Have been getting errors exporting playlists to Spotify [and playing it on Listenbrainz], once the account is successfully connected. By my own account and by other people (so it has been replicated).

Thess playlists always fail:
https://listenbrainz.org/playlist/6b8b5583-46d3-4a4c-8eec-d4e90ed1ed27/
https://listenbrainz.org/playlist/7bbc9a5d-dbf1-44bd-9cc8-97b505ef4a53/
https://listenbrainz.org/playlist/f51de88b-6c00-4e10-b8dd-9b07ea5d899e/
https://listenbrainz.org/playlist/7bbc9a5d-dbf1-44bd-9cc8-97b505ef4a53/

These ones work fine:
https://listenbrainz.org/playlist/d726e8ea-72e3-4652-98eb-f0765b33d5a0/
https://listenbrainz.org/playlist/8da62d37-adc6-4b71-9e96-b81cc1b2d951/
https://listenbrainz.org/playlist/a79c5bc0-7f31-4648-88f1-ffa00e1e4e13/
https://listenbrainz.org/playlist/b71f8d72-2a80-46a6-9214-af22898ad405/
https://listenbrainz.org/playlist/7515a9c2-06ad-4588-bd89-b72ec55b4b66/
https://listenbrainz.org/playlist/da6c690b-c533-4f62-bffb-671e9d81fd43/
https://listenbrainz.org/playlist/8abf5d92-f4c2-41d1-9378-6380b5703616/

I have found adding a single track to this playlist:
https://listenbrainz.org/playlist/59d3be13-46c2-4378-8cc1-f6d8a210ee17/

i.e. going from 116 tracks to 117, makes spotify to fail.
You have such example here:
https://listenbrainz.org/playlist/546c371a-2274-4cce-8e87-abdb67860b28/#

Although one of the working playlists at top has 133 tracks and works fine
https://listenbrainz.org/playlist/7515a9c2-06ad-4588-bd89-b72ec55b4b66/#

This is the error in all cases:

The error at console:
POSThttps://api.listenbrainz.org/1/playlist/7bbc9a5d-dbf1-44bd-9cc8-97b505ef4a53/export/spotify
[HTTP/2 500 Internal Server Error 656ms]

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://api.listenbrainz.org/1/playlist/7bbc9a5d-dbf1-44bd-9cc8-97b505ef4a53/export/spotify. (Reason: CORS header ‘Access-Control-Allow-Origin’ missing). Status code: 500.

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at https://play.google.com/log?format=json&hasfast=true&authuser=0. (Reason: CORS request did not succeed). Status code: (null).

The successful playlists work without these errors at console.