Sounds like either the file or a field's index may be damaged.
You can bring up your field definitions, select a field and turn indexing on and then off to rebuild the indexing on that one field. You could import all your data into a clone and that will rebuild all the indexes in your file.
Note: file corruption could be messing up the indexing. In that case rebuilding the database may be only a temporary fix. If the problem returns or persists, you may have to replace your file with an undamaged copy.
If re-indexing your fields does not work, you can recover the file and test it to see if it works as expected. If the recovered file works, replace your file with an undamaged back up copy if you have one.
Incredible what a powerful mind can achieve.
The idea of a corrupted file would never have entered my mind.
Everythings works fine now.
Thank you very much.