Modeller cannot identify the 501st residue in the first sequence in your
alignment. If you look at your alignment, the first sequence is 2hyd,
and if you look at the sequence around position 501, it is:
TATDEEV VEAAKMANAHDFIMNLPQ
A space character is not a valid residue type. Remove the space between
EEV and VEA and the problem should go away.