Cannot add item to custom collection

[fritzmg]

Have you restarted your Jellyfin server recently?  Some users with strange issues were able to resolve the issue by simply restarting Jellyfin.

I have just restarted the container, but the behavior has not changed.

What OS?  If Linux, which distro?  Are you using docker?  If so, which image?  The official or linuxserver image?

The Jellyfin server runs as a docker Container on a Synology DS220+. It is the official jellyfin/jellyfin image (the latest, i.e. 10.10.3 at the moment).