Running Unraid 6.8.3 I was receiving "database locked" SQLite exceptions whenever Sonarr or Radarr tried to import a completed download. I tried numerous things from ensuring the dockers were only on a cache drive, to checking my paths, etc. I believe I finally found the issue after stumbling on this thread for pre-release testing: SQLITE DATA CORRUPTION TESTING.
Following the advice in this thread I changed Settings/Disk Settings/'Tunable (scheduler)' to 'none'. Since I had taken the array down I also took the opportunity to re-format my cache drive to XFS. Everything seems to be working perfectly now, no errors in Sonarr and Radarr with numerous downloads completed.
I wanted to submit this because the reference thread mentioned this was an issue for SQLite databases on the array, however I was observing the same issue on my cache drive. My Unraid has undergone several upgrades from all the way to 4.x so my situation could be a corner case due to old settings, etc. Anyway, just wanted to report this in case it is helpful, and especially in case others hit this as I spent far too much time trying to figure this out (and re-installing all my dockers 😪).