Thanks again for your many ideas!
Let's start out with a lightning round:
DennisN said:
- infighting among crew
- disobedience
- mutiny
- food/water shortage (turn it into cannibalism if you want to go wild
)
Well, that's what the main story is about 
(not the cannibalism, but people anticipate food shortages and rationing once the population increases beyond the ship's intended capacity

). But what we're talking about right now is intended to be the mid-point plot twist. In other words: An external, physical problem that causes of amplifies all of the psycho-social problems above.
DennisN said:
I'm already worried about people misreading my story as an allegory on the pandemic, when it's not intended as such, merely because some of the ethical questions overlap

(specifically with regards to bodily autonomy). So having disease play any major role in the plot (with all the expected consequences, i.e., quarantines etc.) would only increase that risk.
The crew are aware of the "War of the Worlds" risk, though (meaning, the danger of infection once they land on the new planet). Which is why they do have a biolab on board to constantly create artificial viruses, in order to keep everyone's immune systems sharp. Yes, that feature of the setting could serve as a setup for a sub-plot about a virus (especially because I mention it in the very first chapter). But so far, this is just a red herring.
DennisN said:
accidents (e.g. I think there was a famous ship that hit an iceberg

)
Guess what: The parallels to the Titanic have indeed been there from the beginning of the story - from the title song I wrote for it, all the way up to a scene in which Titanic is shown during movie night.
That's why I've always liked / still like any type of accident that includes a collision. Only in this case: With a particularly small object. The problem is that, at 0.125 c, even collisions with small particles might still result in the equivalent of the Titanic ramming a nuclear bomb instead of an iceberg. That's why I'm wondering how small such a particle would have to be in order to only cause smaller levels of damage to the ship.
Titanic as a story for a movie was only interesting because some people managed to survive, and thus, the ethical conflict arises
who gets to survive. If all passengers had drowned together with the ship, there wouldn't have been any dilemmas or choices left to make for the characters.
DennisN said:
fire breaking out onboard
This is indeed a very flexible solution, although it would probably only lead to a few casualties, given the size of the ship.
The most damage a fire could do would probably be close to the hydrogen tanks.
I already have a scene in which one character gets burned from a leak in one of the hydrogen tanks. The question is how much hydrogen has already escaped into the surrounding spherical tank at that point, and how I can prevent that spark from setting the entire sphere on fire.
DennisN said:
piracy (though it may not be applicable to your story?)
Indeed not applicable, as there are no aliens in my story. Well, perhaps some primitive animal life forms on the target planet at the very end, given that the whole point of the journey is that this is the most Earth-like planet we know of so far: Teegarden b. Which answers the next question:
gmax137 said:
125 years (Teegarden b is 12.5 light years from Earth).
Originally, I simply had the ship travel at 0.1 c to make this equation work. But that would have required the ship to accelerate to this coasting speed pretty quickly (i.e., probably at g forces too high, especially since the ship has rings for artificial gravity, not a "skyscraper-like" structure, as it has been proposed for constant-acceleration ships). Constant acceleration also wouldn't provide any gravity anymore once the ship has reached its coasting phase - which will be the majority of its voyage no matter what.
Therefore, we decided here on the forum that 0.125 c would work better: A slightly higher coasting speed (travel at full speed for 75 years), but a longer acceleration and braking phase of 25 years each.
256bits said:
So how does the ship save itself from debris after the ship has turned with the defense mechanism on the now rearward of the ship.
Drakkith said:
I was under the assumption that the protection (lasers + shielding) was on both the fore and aft portions of the ship.
Indeed, there are deflector lasers on both ends. Part of the reason for this is also that, if the ship had to send a probe back to Earth, such a probe would be equipped with solar sails - and then, the lasers on whatever end of the ship is currently facing backwards could be used to accelerate that probe (the same concept as Breakthrough Starshot).
The expected use for this was of course if something happened to the ship - then at least the crew could send a last "message in a bottle", even in physical form, if regular communication (radio waves etc., which travel at light speed) failed. Instead, it's Earth which suddenly stops communicating about 2/3rds through the story (meaning, in reality, they've stopped sending signals about 10 years ago). One officer proposes to launch the probe to see what's up at home - but that probe would take 50 years to get there (10 light years at 0.2 c), and they would only get the first data after 60 years. Hence, the commander decides not to expend the materials for the probe, since any matter that leaves the ship is lost forever.
256bits said:
Devils in the details - maybe budget constraints, or sky is the limit funding.
256bits said:
Of say ten ships launched, yours might be the unlucky one.
Those two points seem mutually exclusive to me: If there was enough funding to build several generation ships (an "Exodus fleet", as Isaac Arthur called it - which is coincidence, because my ship is called "Exodus"), it would be a hard sell to make the reader believe that somehow, there wasn't enough funding to equip them with lasers on both ends.
Or rather:
Why send ten ships with lasers only on one end, if you could send five with lasers on both ends? Especially since the latter increases the chance of survival for the individual crews? After all, human beings are K-type strategists, not R-type.
Hence, in my case,
the expenses to build even just a single generation ship are already so literally astronomical that only the richest man in the world can even afford to get the project started (he still needs to recruit a bunch of investors for it, who mainly profit off of the marketing for the recruitment campaign - because of course, the ship itself will leave and not come back). So there is only one single generation ship - for the time being. There are plans to build further ones, but they will be sent to different star systems (Tau Ceti, 82 G. Eridani, Trappist-1, Ross 128 etc.).
Stories about Exodus fleets can show political / inter-crew conflicts between the various ships of that fleet (such as in Adam Oyebanji's "Braking Day"). However, such stories tend to require travel between the ships, perhaps also spacewalks (both are featured in "Braking Day", at least). And for all I have heard, spacewalks while coasting at relativistic speeds would be a non-starter (again, mostly due to radiation).
For the same reason, my entire ship already has no windows, only cameras on the outside - and screens on the inside that show what these cameras show (like "the screen" in Star Trek). But every spacesuit has a visor.
DaveC426913 said:
After all, it might now be more efficient to leave the ship in the original orientation and just move the propulsion component (which I think might have been SI's original plan).
I think we even discussed the possibility of the ship having two drives? One in the fore sphere, one in the aft sphere? That would eliminate the need to move anything around, or turn the ship around. And it would also make sense, because redundancy is a huge asset, if not a necessity: Anything that breaks must be replacable or fixable on-board. There are no possibilities for a repair stop.
Conversely, that's why, whenever I need to get rid of some technology for plot reasons (artificial wombs, CRISPR, sperm bank with extra genetic material from non-crew members etc.), it's safer for me to postulate "the ship started with these things, but they got destroyed along the way". And then, the safer explanation for that is human sabotage, rather than an accident. Because accidents rarely destroy things completely - properly planned deliberate destruction can.
Basically,
I always blame it on the one youth rebel organisation from Generation One (the first people born on the ship) whenever I need an explanation for
why something has been destroyed that the ship used to have at the beginning

. They had every incentive to make the mission fail, because they wanted to make the ship turn around and return to Earth before it had reached its full coasting speed (which, as the relativistic calculator told me, coincided with the ship passing the Oort cloud).
After that, braking would simply result in the ship getting stranded in interstellar space (because the remaining fuel was only intended to initiate braking once the ship approaches the target system).
Sure, the primary concern with anything I simply add to the ship is mass. But if there's one thing I have to handwave away, it's that. Not by pretending it doesn't matter, but by simply never specifying how much exactly my ship weighs.
I mean, I'm already content that the layout / internal map of my ship is comparatively clear-cut. Does anyone know exactly what the Enterprise looks like from the inside? Or the exact layout of Hogwarts?

(Trick question: it is established in the books that the interior of Hogwarts changes constantly, and a corridor that exists today may no longer be there tomorrow - because magic, duh.)
The worldbuilding of my ship is fairly "hard", in the sense that, once you understand the layout, you know exactly where characters can and cannot go. I can't just make up a new area on the ship out of thin air, whenever the plot would need me to.
That's why I've started digging deeper into the specific construction of the ship's central trunk (around which the rings rotate) in the neighbouring thread.