When filtering by form I suddenly get the words: Is Null / Is Not Null appearing in the drop down menu of all fields.
I can then no longer filter at all?
This is carried over to filter by form too!

When I export, or cut and paste the records into my backup database (filters working ok before I do!) the same thing happens!

Can anyone help….please?

Set the Filter Lookup property to Always. you can find it on the Data tab for each control.

Also this is a Forms issue, not a tables issue