Just tried to run the project and attach it to the project from Rider, but the Unity is stuck on Reloading Domain.
Tried several times, but no luck.
Reboot? Sometimes this helps.
Rebooted, and removed the Library folder (this forces to fetch and rebuild everything).
I have to wait ~40 min to complete all these.
No luck.
For honesty’s sake, during the last ~ two years, this worked for me only once. I don’t remember which version of Unity, but I’ve managed to run it.
Also, looking at this all, it seems that using a Mac is unfortunately not a good idea, since all the time Mac OS versions look most unstable.
For the Unity 6 Preview, the last stable version was 6000.0.3f1. Where all worked as expected. (Script debugger not tested, as all worked fine 
Thank God, I’ve created a new branch before trying the next builds.