This is exactly like https://github.com/transmission/transmission/issues/8123 I figured I'd wait for transmission-daemon 4.1.1 to be in Debian before reporting a bug, in case it was solved already. But it isn't. To reproduce: "click on a magnet link in a browser that has transmission registered for magnet links". It will show "Unable to add torrent". Same problem from command line: fragments "magnet:?xt=..." However, copy the link, open Fragments, and it recognize the magnet link from clipboard.
There is a pending upstream patch here: https://gitlab.gnome.org/World/Fragments/-/merge_requests/202