This problem has been happening to me in prior versions as well.
When Sonarr downloads an nzb file from one of my indexers Newbin can only repair it about 50% of the time. Same thing happens if I do it manually. It's not a problem with the indexer as no one else is having the same problem and it's not a problem with Sonarr.
I can always successfully repair if I go to AutoPAR/Run QuickPar
If I then go to AutoPAR/UnRAR/Join Files it comes up with this message : Quick UnRAR:No filename found
If I click on the files properties and hit refresh that doesn't help either.
The only way I can unrar it is if I use Winrar.
This is a bit annoying so I was wondering if anyone else has encountered this problem and if they know a fix?
BTW am using Windows 10 but it was happening in Windows 8.1 too.
Cheers