The out-of-the-box configuration of Plex on FreeBSD causes the localmedia agent to barf when trying to read subtitles containing Unicode characters.  In "/usr/local/plexdata/Plex Media Server/Library/Logs/PMS Plugin Logs/com.plexapp.agents.localmedia.log" you get something like:

CRITICAL (agentkit:1057) - Exception in the update function of agent named 'Local Media Assets (Movies)', called with guid 'com.plexapp.agents.imdb://tt1617661?lang=en' (most recent call last):
  File "bundles-release/Framework.bundle-dist/Contents/Resources/Versions/2/Python/Framework/api/agentkit.py", line 1055, in _update
  File "/usr/local/share/plexmediaserver/Resources/Plug-ins-f38ac80/LocalMedia.bundle/Contents/Code/__init__.py", line 56, in update
    localmedia.findSubtitles(part)
  File "/usr/local/share/plexmediaserver/Resources/Plug-ins-f38ac80/LocalMedia.bundle/Contents/Code/localmedia.py", line 260, in findSubtitles
    if os.path.isfile(os.path.join(path, file_path_listing)):
  File "/usr/local/share/plexmediaserver/Resources/Python/lib/python2.7/genericpath.py", line 29, in isfile
    st = os.stat(path)
UnicodeEncodeError: 'ascii' codec can't encode characters in position 43-44: ordinal not in range(128)

This can be fixed by editing the startup script "/usr/local/etc/rc.d/plexmediaserver", changing the lines:

export LC_ALL="C"
export LANG="C"

to

export LC_ALL="en_US.UTF-8"
export LANG="en_US.UTF-8"

This fix is described in the Plex forums here: https://forums.plex.tv/discussion/comment/780533/#Comment_780533