The code that spits that error is trying to do some string name matching of the four-character PDB stem of a template pose versus whatever’s being aligned. I would guess you’ve got a line in your alignments with the wrong PDB name (doesn’t match the PDB file names, maybe? It appears to be matching on the first four characters.).
There are some “debug” level output statements nearby, re-running with the verbosity level high may help. (I think that’s flag -out:level 500 but I’m not sure.)