Can't add to existing collection (list of collections empty)

[Sagrafin]

When adding an item to a collection, the list of existing collections is empty. The only available entry is New.. (despite other collections existing (I can view them, delete items from them, etc)).

I tried entering the name of an existing collection when selecting New..., but that just overwrites the existing collection.

I did have the TMDb Box Sets extension installed. But when I disable it, I get the same result.

I haven't seen anything relevant in the logs or the browser console. I'm not sure which HTTP request is responsible for fetching collections, so I don't know if the server returns an empty list, or if the client (I'm using a regular web browser) just doesn't display it right.

Jellyfin: 10.10.3 (linux docker)
Browser: I tried it with firefox and chrome