I had the same error and was despairing. Now I have found the solution to my problem. It was a login entry that was faulty.
To find this in 300 entries, I created a new vault locally on the computer and copied all entries into it. After that I deleted all entries and restored them from the trash in groups of about 20 entries until I found the faulty one. This one could not be restored and did not show any entry.
In the work vault, I could only delete this entry in the mobile app, not on the computer.
After it was finally removed, the export as a .json file worked.
In this respect, this problem can hardly be solved by the developers, but only by the respective user himself.
I hope this helps at least some of you.
