Trying to save a .wma file (should be moved to a different folder though) but I get a red error sign at the filename.

Also clearing tags etc. and then trying to save does not help.
Googled around since 1,5h but no success.

Switched the verbosity in the log to debug, here is the output:

D: 00:21:14,859 /usr/lib/picard/picard/file.update:500: Updating file <File 'cde.wma'>
D: 00:21:14,860 /usr/lib/picard/picard/formats/asf._save:188: Saving file '/mnt/DataAndBackup/Music/Kaerntner/bla/cde.wma'
E: 00:21:14,861 /usr/lib/picard/picard/util/thread.run:51: Traceback (most recent call last):
File "/usr/lib/picard/picard/util/thread.py", line 47, in run
result = self.func()
File "/usr/lib/picard/picard/file.py", line 218, in _save_and_rename
new_filename = self._rename(old_filename, metadata)
File "/usr/lib/picard/picard/file.py", line 380, in _rename
self._make_filename(old_filename, metadata))
File "/usr/lib/picard/picard/file.py", line 349, in _make_filename
new_filename = self._script_to_filename(naming_format, metadata, settings)
File "/usr/lib/picard/picard/file.py", line 306, in _script_to_filename
if isinstance(metadata[name], str):
File "/usr/lib/picard/picard/metadata.py", line 289, in _getitem_
return self.get(name, '')
File "/usr/lib/picard/picard/metadata.py", line 284, in get
return self.multi_valued_joiner.join(values)
TypeError: sequence item 0: expected str instance, int found

D: 00:21:14,867 /usr/lib/picard/picard/file.update:500: Updating file <File 'cde.wma'>