I had this error repeatedly in my subzero log file:

#1679 | 2019-11-03  22:54:16,636 (70000319d000) :  DEBUG (config:335) - custom check failed  for: /Users/lumiere/Library/Application Support/Plex Media  Server/Plug-ins/Sub-Zero.bundle/Contents/Libraries/MacOSX/i386/UnRAR/unrar
-- | --
#1680 | 2019-11-03 22:54:16,701 (70000319d000) :  DEBUG (config:335) - custom check failed for: unrar
#1681 | 2019-11-03 22:54:16,702 (70000319d000) :  WARNING (config:345) - UnRAR not found

unrar is installed on my system and correctly included in environment path both globally and for my logged in user. So, I did not get how this error message was there. Until I read the log lines more closely and realised that the unrar file within subzero is not a script to check for unrar as I had assumed. It is an additional unrar executable which subzero insists on using, ignoring the system unrar.
Problem is, the "subzero specific" unrar file is not set to be executable:

$ ls -lah /Users/lumiere/Library/Application\ Support/Plex\ Media\ Server/Plug-ins/Sub-Zero.bundle/Contents/Libraries/MacOSX/i386/UnRAR/
total 640
drwxr-xr-x  3 lumiere  staff   102B Oct 26 05:18 ./
drwxr-xr-x  3 lumiere  staff   102B Oct 26 05:18 ../
-rw-r--r--  1 lumiere  staff   319K Oct 26 05:18 unrar

So. Now it makes perfect sense subzero is complaining.

I ran:
$ chmod ug+x /Users/lumiere/Library/Application\ Support/Plex\ Media\ Server/Plug-ins/Sub-Zero.bundle/Contents/Libraries/MacOSX/i386/UnRAR/unrar

This fixes the issue. Verified error messages are gone from logs.
But unless fixed within subzero, it likely needs redoing manually on each subzero update.