Hmm... which mac OS version do you use exactly? Maybe you could send a report when this happens. To do that, try to join a server which triggers this message, then type "report" into console and send the report.
If you try to join your own server, it could be extremely helpful if you could also send us a log of the server as well (which covers the moment you tried to join on a Mac)
I have 11.6 Big Sur, maybe I should try updated to Monterrey and see if that fixes it. I sent a report when trying to connect to my server. The servers that it gets the error message on the loading stops at 5%.
On my server console it doesn't even register that I tried to connect.