I'm certain that second one should work... I've found perhaps two dozen other communities that have the same problem. Meanwhile, dozens of others work fine.
Edit: @[email protected] suggested I try searching for the community first. I had actually tried this but it didn't work, which is why I started trying the deep link approach above; that worked for some communities.
Turns out the deep link by itself will not discover new communities, only searching for them will, and the search can take a long time and will show "No results" for a little while.
So if you're experiencing this, search instead for [email protected] from the remote instance, then the deep link will start working.
You can search by URL instead https://lemmy.ml/c/kubuntu
it will show the community in results
Using [email protected] could take some seconds, and you need to search again for community name only without the instance.