Reported on the forums:

"I’m trying to import my playlists over via the Web UI but it seems to be failing every time saying that an unknown error has occured. Is this new or is it maybe known and there are ways to proceed?

I saw there was a similar post, but that one seemed to mention large playlists. I accordingly tried to import a tiny playlist and I have the same error pop up for it as well. So I don’t think it’s related."

Other users inthe thread confirm this is an issue, and not related to being big playlists

https://community.metabrainz.org/t/an-unknown-error-has-occured-when-importing-any-playlist-from-spotify/813962

The code to import playlists from Spotify lives here: https://github.com/metabrainz/listenbrainz-server/blob/0071abaf853c28c2d237302d643aed31d7e848b9/listenbrainz/webserver/views/playlist_api.py#L970-L974