Docker container running on a Synology. Version 10.8.10.
A stream without username/password (e.g. those in https://github.com/iptv-org/iptv) work just fine. Just this stream I encountered that contains a username/password doesn't. And like I stated in OP the stream itself probably isn't the problem, as KODI can play and list all the channels within just fine.
I prefer to use Jellyfin because it transcodes even these live TV streams for my mobile devices when reception isn't that good.