You did not mention which version you are using. If you have FMP Advanced, you can look for any missing script references in the Database Design Report ("Script: <unknown>")
A couple of ideas come to mind:
• Check the File>File Options… and ensure that there is no script trigger set for 'OnFirstWindowOpen". You may have deleted the linked script.
• Check that you are not calling a script that is stored in your data file before it is opened by the UI file. You may have forgotten to import a script into the UI file.
You have lost an External file refernence since you said "Separation Model". My guess is your "error" version has an External File name change or directory change or even a root directory name change since your saved working version.
Tip: When working with the Separtion model or External Files that share among many DB's. Keeping them all in the same "folder" is best.
When an external file name is changed or the file is moved, FMP gives the error message:
'The file "…" could not be opened. (Not Found)"
and not the message in the screen shot of the OP.
Found it! Thanks Jade. You pointed me in the right direction. I searched all the layouts and found a deleted script trigger for one of them. Deleted it - problem solved.