I cannot open a file from ID CC 16.1 in 15.1 (2020)...

I cannot open a file from ID CC 16.1 in 15.1 although I´ve downgraded it to an earlier version that should work. A rough translation of the problem description reads as follows:

"It wasn´t possible to open Service manual_XXX since it has been saved by a newer version of Adobe Indesign (16.1). You have to use that version or a later one to open the file. To be able to later open it in this version you should choose Save as Indesign CS4 or later, or export it to IDML."

I have tried all of the above without success, the message does not go away, I have to turn the program off via the Activity handler (correct word?). It didn´t work to upgrade the program to 15.1 or restart the computer.

The error message (in swedish) can be viewed below: