Phew, lucky there is already a bug-Report. I thought it was my fault.
In my case it seems like there is a corrupted method within the „db“ build/read/store process. (Since I am on Safari mobile there could also be a little issue with GET. So it might not reproduce the exact same error as it would with chromium/FF based browsers).
If you're still having a problem importing files, is there a chance you could DM a non-working file to the Dev? I think they believe this issue is resolved.
I've solved the original issue already, but it sounds like you've found another (related) issue. If you answer each of these questions, I will hopefully be able to solve it:
Can you please share a screenshot of the error?
Which OS version and Safari version are you using?
Does the import fail if you use incognito/private browsing mode?
Does the import fail for newly created export files (e.g. open incognito, create a chat, export it, and then try to re-import it), or do you only get the error for old export files (e.g. ones you created a week ago or more, roughly)