The only time I have ever seen this happen is when someone has been trying to work with their data on a Floppy DIsk and NOT a Hard Drive. The reason being a FLoppy can not open enough files at one time.
I would suggest you uninstall PAF, then shut down your computer, then reinstall PAF 5.2.18.
Run the Check Records option and see if the pointers have become corrupt.
It is always a good idea to have 2 beckups and alternate them.
Never do a backuo with first running check records.