Well as commonly occurs when troubleshooting, reading over my written statement of the problem posed a potential solution..... for some reason PBP can't open "S:\Main1.2.pbp", but it can open "C:\Main1.2.pbp". It can, however, open "S:\Main12.pbp". I have no idea why PBP is picky about extra periods when the file is on the S drive but not the C drive. Any suggestions as to why this could be?

It's an easy problem to avoid, but I'd still like to know why it is that PBP has this issue.