I use Mathematica for MAC (18.104.22.168). My front-end application hangs on start-up. There is no splash screen but the Activity Monitor reports the process as non-responsive.
I've found an older thread reporting a similar problem: Front end freezes on start-up
That solution works for me as well. I have to delete the "pacletSiteData_11.pmd2" file from "~/Library/Mathematica/Paclets/Configuration", then the front-end application starts without problems.
I'm just surprised that the problem occurs — the original thread dates back to 2015.