I have some FLAC files I can't tag. When I hit Save in Picard, the NoEntry symbol () appears next to the tracks. The log has this:

I: <sip.voidptr object at 0x0361C0C8> 22:27:41 Plugin 'Cover Art Downloader' from 'C:\Program Files (x86)\MusicBrainz Picard\plugins\coverart.pyo' is not compatible with this version of Picard.
I: <sip.voidptr object at 0x0361C158> 22:27:41 Plugin 'Original Release Date' from 'C:\Program Files (x86)\MusicBrainz Picard\plugins\originalreleasedate.pyo' is not compatible with this version of Picard.
I: <sip.voidptr object at 0x0361C158> 22:27:41 Plugin 'Local Cover Art' from 'C:\Program Files (x86)\MusicBrainz Picard\plugins\localcoverart.pyo' is not compatible with this version of Picard.
I: <sip.voidptr object at 0x0361C158> 22:27:41 Plugin 'Add Cluster As Release' from 'C:\Program Files (x86)\MusicBrainz Picard\plugins\addrelease.pyo' is not compatible with this version of Picard.

E: <sip.voidptr object at 0x03646128> 22:28:04 Traceback (most recent call last):
File "picard\util\thread.pyo", line 59, in run_item
File "picard\file.pyo", line 154, in _save_and_rename
File "picard\formats\vorbis.pyo", line 144, in _save
File "mutagen\flac.pyo", line 627, in save
File "mutagen\flac.pyo", line 66, in writeblocks
File "mutagen\flac.pyo", line 459, in write
AttributeError: 'NoneType' object has no attribute 'encode'

I haven't come across this in any of the other FLAC files I have, just this one release I purchased off Bandcamp. Example file temporarily uploaded here.