What is the error message? "causes an error" tells us nothing.

Here is the error message.

When you click okay, it opens up in the code editor window with the Open event high-lighted.

OK. Compact & Repair your database. Then Compile it and try again.

You could also try going to File --- Save As and saving the database as a .accdb file.
Does the same error appear?

So, I hadn't thought of the compact and repair option, yet. I tried saving it as an .accdb file. I got to the point where it asked if I wanted to delete all the VBA code. And I did that after making a backup. Then I copied in the Open even code and got an "User-defined type not defined" error that stopped on Dim db as Database. So I Googled it and it said I needed to make sure the DAO reference was turned on. But that gave me an error loading that in the Reference library. So, I created a new BLANK database, then I imported ALL of the old database objects. And then it works. At least the forms open and the code seems to work. I can run Debug & Compile with no errors. So, I think it was something between the old Access format and the new Access 365 that was giving issues. And my experience with Access. Sometimes the code it stops on and/or the error it suggests isn't always what is causing the problem.

Yes, that's what I was going to suggest. You should not need a reference to DAO in the newest versions of Access. That was an old problem when they switched to ADO as the default. I would create a brand new database file like you have and then import all of the objects into that.

Not to knock Kevin's comment, ""causes an error" tells us nothing."
But the MS error messages didn't help much either.
Bahahahaha

Yeah, but at least seeing the meaningless dialog that MS gives us is better than nothing. ;)

@ Richard, True. Now we know Gary wasn't making it all up.