I've just installed Unity 2019.4.20f1 and Rider 2020.3.2, and created a new unity project.
I set the external editor to the rider executable (in /usr/bin/rider
in my case)
I opened the sln in rider and it installed the rider plugin.
Now with both open (and having tried opening both with the other), rider refuses to connect to unity (shown by the icon down the bottom).
It should show connected but hovering just shows the error in the title. Play button in Rider is greyed out.
I've checked the first page of google and nothing there seems to fix the issue.
Steps to reproduce:
Update: Just tried Unity 2020.2.5f1 with the same issue
Installing Rider using the jetbrains toolbox app instead of the AUR results in a correctly working installation.