I wanted to update this thread because I've recently had the exact same problem and came by a solution that allowed me to continue printing.
To cut a long story short it was the same issue - suddenly all my prints failed shortly after starting with grinding. Although later I did establish that prints with a much smaller X/Y travel worked fine.
I re calibrated everything, tried different high quality filaments, replaced Bowden tubes and nozzles and used both extruders. (UM3+).
I came by this thread and was intrigued by the proposed route cause - high temperatures and high humidity.
I live in Scotland and while the summer has been good so far it CAN NEVER BE TOO HOT IN SCOTLAND!
However the temperature was 20+ degrees Celsius.
After pursuing a mechanical issue as the cause I hit a dead end then some things came together:
I then thought back to the ambient temperature issues in this thread. Yesterday when I replaced the Bowden tubes it was ~25 degrees and very humid. The hottest day of the year so far.
This reminded me also of a comment UM support had made about pooling melted fluid in the nozzle. When this problem first occurred I noticed immediately that the ends pulled from the nozzle were unusually thick, basically the same dimensions as the metal tube. And then I realised that this was because the filament had fully melted.
I'd been surprised that I was having the same issue on both extruders so it couldn't really be the Bowden tube as ext 2 has hardly been used.
So I then reduced the print temperature from 200 deg to 185 deg and the problem was solved. I've since upped the build plate from 60 to 65 degrees as there was some minor adhesion issues (first layer bubbling). But since then it's remained hot outside (relatively speaking) here and I've done 3 major prints with no issues.
But I'm still not sure why my UM2+ in the same location is merrily printing away at an unchanged 220 degrees...
Thanks for all the contributors to this thread. TL;DR I solved this issue by reducing the print temperature from 200 to 185 degrees C.
1. Failed print.
2. Jammed in tube guide for extruder 2.
3. Thickened end after melting.