As reported by sobornost in the forums, here.

"I have been trying to export my weekly mix to Spotify. Unfortunately, it fails with the following error:

Error
An unknown error occured.

This is a different error from this issue that’s why I’m creating a new topic"

sobornost has identified the issue as the description being too long for Spotify.

We can shorten the LB description length, or we can truncate text when exporting to Spotify.