Wrong fop error

On some machines, LWW Personal 5.0.1 can't load fasl files and signals  
the following error:

Wrong fop (99) while reading fasl file

(the number varies, another one is 117)

I tried to google the error message but without any success. The fasl  
files are created on the same machine by the same LW which then fails  
to load them. Does anybody know what is going on? LW is quite unusable  
on these machines.

Thanks,

Michal