You'll need to examine two things:
The relationship on which the looked up value setting is based and the settings specified in the Looked up value field option to see what isn't set up correctly.
When checking the relationship, make sure that the table occurrence referenced in the looked up value setting refers to the correct data source table. Since you've been moving table from one file to another, it's possible that the wrong data source table is being referenced.
Likewise, you may find that your auto-enter settings on the field no longer point to the correct table occurrence after merging the tables, so you'll need to check them also.
Good to hear from you. You got it! It was a problem in the relationship between the table occurences. I 'simplified' the relationships between tables and apparently I did it wrong. I reconstructed the relationships like the original file and now things are working again. (I still like to simplify things when I've got time, but I'm fighting this dead line so cosmetic things are to be done later)
Thanks for helping me out!
A Helpful design hint you may already be using:
Save frequent copies of your work and keep the separate copies. Then, if you make a change that you later deside to discard, you have the option of reverting back to a previous copy (or importing/copying elements of it into a new version) instead of trying to remember and recreate what you had previously.
That is a hint I am aware of, but practice not often enough. Thanks for the reminder!
Same for me. I've been toying with using Install OnTimer to save hourly copies of a file under developement with data from the current timestamp appended to the filename so that I have automatic hourly backup copies on file.