Bug fixed in 10.1.0 via paclet update
Since a few days now, each time I open a new Mathematica session I get the small red error popup window with the error messages:
Throw::nocatch: Uncaught Throw[False] returned to top level.
repeated many times, as you can see from this screenshot:
Apart from the error window, Mathematica seems to work without problems. I didn't do anything "strange" with Mathematica recently, no new installation or update or anything of the sort.
What does this error mean, and what are the possible causes and fixes?