One of the many cases where the Lemmy frontend does not display any error message that the backend gives. It's an known issue and likely will not be fixed in the official lemmy-ui frontend as the developers decided to rather re-write it from scratch and only do minimal maintenance on the lemmy-ui frontend.

I think sooner or later we will switch to another Lemmy frontend (current favourite seems to be Photon) as I am not overly convinced that the official re-write will be much better than the current one (as it uses some obscure and not very popular Rust based web-framework).