It sounds to me as though default.fpm may be corrupted. Were I in your position I would zip up everything in gameguru\files and put it somewhere safe, then reinstall GG and try to run it. If it works without generating the error then
go into mapbank and copy default.fpm
then restore default.fpm from your zip file
then try running GG again.
If all is okay then it's not default.fpm but perhaps whatever map you were working on.
If the error returns then delete default.fpm and rename your copy of it.
Then try restoring everything *except default.fpm* from your zip file.
Apologies if I'm trying to teach my grandmother, (which I doubt, given how old I am) to suck eggs.
Good luck.
You can't teach an old dog.