Sorry, sign errors in which equation? By the way, the way it is derived in the link is not necessarily consistent.
What is the correct answer (of the alternatives posted)?
The link provided may be using a different convention. Until now, I have provided all the algebra and the question. It was suggested that "...realize that the link you provided...has the solution...?" The original question has two alternatives, and it is useful to see what the forum members come...
What is the correct way to expand (p3-p4)2 where p3 and p4 are 4-vectors, with metric gmu nu=diag[1,-1,-1,-1], p = [wp, p], where p is 3-vector, and wp= (p2+m2)(1/2)