Because fetching an infinite history slows Sonarr down over time. The best way to deal with it is to have Sonarr remove history items after import, but you can add DownloadClientHistoryLimit to the Config DB table with a higher value.
24 hours later, no new unprocessed download folders seem to lay around in my filesystem. I‘ve deleted some old ones now to clean up. Will see tomorrow, it the good looking condition stays like that.
I can confirm, this solved my problem. I‘d suggest to make this configurable though the web interface, but only if „Remove“ is selected.
I still have some finished download folders, but they do not contain any usable video files. Is there a way to let them „auto fail“ and delete them afterwards so sonarr can try to find another release for the same episode?
Sonarr won’t fail them unless the download client marks them as failed. You might be able to have a post-processing script in your download client do that, but it’s not something Sonarr will do on it’s own.