Went to my production server (Win 10) and changed it from Blackhole to NZB Remote. I did add the $(NZBFile) to the Remote NZB unrar folder (a year ago you said to use a naked path and you appended the $(NZBFile)). Turned on debug logging and exact same sequence of events. I see one error in file creation, maybe you will understand. BTW, with Blackhole I use the same download folder for all videos and Sonarr has no trouble with that.
[10:38:47] DEBUG CheckFilename: Using Path:D:\NZBdrone\Complete\
[10:38:47] DEBUG Spooler: Using Path:D:\NZBdrone\Complete\
[10:38:47] DEBUG Par File added
ummy.File.Name.s02e09.Series.Name.hdtv.x264-w4f.2348D888.vol15+2.par2
[10:38:47] DEBUG CheckFilename: Using Path:D:\NZBdrone\Complete\Dummy.File.Name.s02e09.Series.Name.hdtv.x264-w4f.2348D888.vol15+2.par2
[10:38:47] DEBUG AutoPARPlugin: Good files = needed files and goodblocks = needed blocks[274504]-[FULL]-[#a.b.teevee@EFNet]-[ Dummy.File.Name.S02E09.Series.Name.HDTV.x264-W4F ]-[09/22] - "Dummy.File.Name.s02e09.Series.Name.hdtv.x264-w4f.part1.rar" yEnc
[10:38:47] DEBUG Combining BAD Complete
[10:38:47] DEBUG First Filename:D:\NZBdrone\Complete\Dummy.File.Name.s02e09.Series.Name.hdtv.x264-w4f.part1.rar
[10:38:47] DEBUG UnRAR Temp: D:\NZBdrone\Complete\Dummy.File.Name.S02E09.Series.Name.HDTV.x264-W4F.nzb\ UnRAR Filename: D:\NZBdrone\Complete\Dummy.File.Name.s02e09.Series.Name.hdtv.x264-w4f.part1.rar
[10:38:47] DEBUG CAssembleRAR UnRARPath #1:D:\NZBdrone\Complete\Dummy.File.Name.S02E09.Series.Name.HDTV.x264-W4F.nzb\
[10:38:47] DEBUG CAssembleRAR UnRARPath #2:D:\NZBdrone\Complete\Dummy.File.Name.S02E09.Series.Name.HDTV.x264-W4F.nzb\
[10:38:47] DEBUG UnRAR Error:File create error
[10:38:50] DEBUG NZB Autoload Watch: D:\NZBdrone\Incomplete Folder: D:\NZBdrone\Incomplete\
[10:38:50] DEBUG NZB Scan Folder: D:\NZBdrone\Incomplete\
[10:38:50] DEBUG NZB Scan Folder is empty: D:\NZBdrone\Incomplete\
[10:38:50] DEBUG NZB Autoload No files to process
[10:38:53] DEBUG Accept: application/json, application/xml, text/json, text/x-json, text/javascript, text/xml
[10:38:53] DEBUG User-Agent: Sonarr/2.0.0.3953 (RestSharp/105.0.1.0; Windows/6.2)
[10:38:53] DEBUG Host: localhost:8000
[10:38:53] DEBUG Accept-Encoding: gzip, deflate
[10:38:53] DEBUG SbServer: Read Request GET /api?mode=queue&start=0&limit=0&apikey=newsbin&output=json HTTP/1.1