This error message shows up in the log:
File <filename>.nfo has missing articles. Skipping file since it is not a video.
This occurs for both Radarr and Sonarr, while the setup has worked and was unchanged for a week (read: initial setup).
As a result the item is added to the blocklist and a new search takes place. This then leads to all available releases to be downloaded and one by one added to be blocklist.
*.nfo is part of the files to be ignored. I have tried removing it, adding it again. Restarting also didn't help. When opening the file from the content directory it plays normally