I have some queries in my Access database which pull data from Excel files that appear in the database as linked tables. They have worked just fine, until suddenly and inexplicably I was getting the error "External table is not in the expected format," when trying to access them.
I thought it might have to do with the fact that I was using a macro-enabled workbook, but it was fine before. I do have a mail merge set up in Word which is linked to the database, and using one of the aforementioned queries.
It turns out that the issue was due to the mail merge document. Once I saved and closed the mail merge file in Word and tried accessing the queries and tables again in Access, the error was no longer appearing.
It seems that if a Word mail merge is connected to the database, this error may appear. I am not sure as to why a more appropriate error is not appearing; after testing it seems to happen regardless of whether the linked file is a macro-enabled workbook or not.
In short, as Olivier put it, the file was locked by Word. A simple issue, but not exactly clear given the error message (unless you follow Andre's logic that the expected format is a non-locked file, hahah).
I hope this helps someone else!