>>My application just read and update dbf in 1 mintues intervals. Please advise me on how to solve or what to look for in solving this problem. Thank a lot.<<

Hard to say. If you are just reading and updating a dbf:

1.) Look at the table after the illegal operation -- Are the indexes OK.
2.) If the dbf is part of a DBC, open the DBC exclusively and do a Validate Database command
3.) Any divide by 0 operations possible?
4.) Have you tried the same operation under VFP 6.0 -- I get a lot of illegal operations in vfp 5.0a
5.) What are you updating and how?

Kevin