https://github.com/metabrainz/picard-plugins/issues/245 has introduced a bug when saving a playlist to a different drive than the referenced files.
Version 1.2 is affected, version 1.1.1 isn't

Debug log includes no hints but it's likely due incorrect use of os.path.relpath and/or os.path.commonpath
https://docs.python.org/3/library/os.path.html#os.path.relpath
https://docs.python.org/3/library/os.path.html#os.path.commonpath