Error: 'ascii' codec can't encode character u'\xe4'

I noticed that PMS is not picking correctly some Albums covers during scan. I made some research and those Albums contains some letters, common in Scandinavian language.
I have tons of Scandinavian music but only recently I noticed the issue.
I use Mp3Tag 2.77 using ID3v2.3 ISO-8859-1 but the same problem occurs with the newest ID3v2.4 UTF-8.

Here an example of local agent log:

2016-06-12 13:50:07,153 (-b6ee4c0) :  INFO (audiohelpers:18) - Error getting file details for /volume1/iTunes Media/Music/Azaghal/Arvet II/01 Vihan Päivä.mp3: 'ascii' codec can't encode character u'\xe4' in position 55: ordinal not in range(128)
2016-06-12 13:50:07,176 (-b6ee4c0) :  CRITICAL (agentkit:1067) - Exception in the update function of agent named 'Local Media Assets (Albums)', called with guid 'local://47365' (most recent call last):
  File "/volume1/@appstore/Plex Media Server/Resources/Plug-ins-2bd156c/Framework.bundle/Contents/Resources/Versions/2/Python/Framework/api/agentkit.py", line 1065, in _update
    agent.update(obj, media, lang, **kwargs)
  File "/volume1/@appstore/Plex Media Server/Resources/Plug-ins-2bd156c/LocalMedia.bundle/Contents/Code/__init__.py", line 280, in update
    updateAlbum(metadata, media, lang, find_extras, artist_extras=[], extra_type_map=extra_type_map)
  File "/volume1/@appstore/Plex Media Server/Resources/Plug-ins-2bd156c/LocalMedia.bundle/Contents/Code/__init__.py", line 365, in updateAlbum
    if os.path.exists(file):
  File "/data/jenkins/pms-deps-universal-v2/BUILD_TAG/linux-synology-i686/output/pms-depends-linux-synology-i686-release-b66d59b/lib/python2.7/genericpath.py", line 18, in exists
UnicodeEncodeError: 'ascii' codec can't encode character u'\xe4' in position 55: ordinal not in range(128)