Thanks for the debug version.
The plugin will detect and translate "\" to "/" internally, so that should not be a problem (at least, it wasn't until now).
Here's the output from the debug version, when double clicking on an item in the playlist (and getting the error message on-screen):
2021-08-31 10:41:04 - get playlist files for:Subsonic:\1
Subsonic:\Music\Blues\The Blues Brothers\03-Gimme Some Lovin'.MP3
Subsonic:\Music\Metal\In Flames\In Flames - I, the Mask (2019)\In Flames - I, the Mask.mp3
2021-08-31 10:41:06 - get stream for:Subsonic:\Music\Blues\The Blues Brothers\03-Gimme Some Lovin'.MP3 --> Music\Blues\The Blues Brothers\03-Gimme Some Lovin'.MP3
null stream
2021-08-31 10:41:06 - 10.0.19042.0 - 3.4.7913.33261P - url: Subsonic:\Music\Blues\The Blues Brothers\03-Gimme Some Lovin'.MP3
System.IO.FileNotFoundException: Unable to find the specified file.
at #=zGAA6s8sNeIk2s8e3MbgPKBc=.#=zHgvYRBZz03zp(#=z7m_eXApVW6Fi #=zDDvHhv8=, #=zOq7nVsKmVAN45X2dUhPXHjw= #=zobVaPak=, Boolean #=zUniNyG_$2MCsxXI5Pg==, Boolean #=zt7g6WGS26h0Y, Boolean #=z_v7nJkCerFdL, Boolean #=zofFkpTY$b_cY, Int32 #=zUUctO58OuoIE)
at #=zGAA6s8sNeIk2s8e3MbgPKBc=.#=zT8yqmiXjHpYk(Object #=zziQJQTM=)