I had a similar situation yesterday with 4.2.0 installed.
So I uninstalled it, and went to 4.2.1.
It worked all day yesterday.
This morning, went to go back in to Cura, and 4.2.1 gets stuck with the "Updating configuration....".
Now after a reboot, it appears to be ok.
Look at the log file it creates during loading.
%AppData%\cura is the location of mine.
Check out stderr.log