May I ask what I should look for in the log files to detect this (and so I can configure fail2ban correctly)?

This is a really great ELI5 explanation of how Usenet filesharing works technically, nice!

Have a look at my German Usenet beginners guide: https://github.com/PCJones/usenet-guide

Let me know if you need any further help

For all German speakers I have a beginners guide: https://github.com/PCJones/usenet-guide

I also use a Hetzner Storagebox for my Jellyfin instance, I mount it via SMB. Works great.

For German stuff there is xrel.to

Usenet is the secret for German stuff. I have a German beginners guide: https://github.com/PCJones/usenet-guide

fileleechers is a board and not an indexer though right? So you can’t add it to Sonarr and Radarr

Feel free to correct me, I don’t know a lot about them since they are invite only

If you are looking for German (or German + English dual language) content it can be very hard to find stuff on public torrent trackers and it’s pretty hard to get onto private German trackers - but don’t worry, there is a solution:

Usenet and the indexer sceneNZBs.com that specialises in German releases have got you covered!

If you want to automate the search for German Dual Language content using Radarr/Sonarr I made a guide (that also works for torrents too): https://github.com/PCJones/radarr-sonarr-german-dual-language