I have also been getting this error with Radarr, Sonarr, Deluge, and Plex then when the server tries to run the mover it will spit out this error.
I have 120 Threads, 128Gb ram and, 2 2Tb NVME BTRFS cache, with 5 16Tb data, 2 16Tb Parity.
The Cache has been running around 90+% full and now I'm having this message with huge Lags and lockups.
Is there a was to increase the "max_children" setting to more then 50?