I've figured it out. The GRTensorII webpage says
You have to use the maple.ini file they are talking about because it contains commands to load all the libraries at startup. The ini file that comes with the GRTensor package loads only the library with the main commands grii.m.
Alternatively, you can use the old maple.ini file but load the libraries you need manually. The Maple command to load basislib.m is
or you can use the GRTensor command
After that everything works normal.