does not run on the PC A and on the other, PC B everything runs smoothly. Both PCs use the exact same file synched over a cloud service
I get no error or anything, it is simply stuck. Before I updated to ubuntu 20.04, this february, also PC A had no problem with the file
There should be no problem with that code.
After updating, did you update ngsolve as well?
Can you run ngsolve in a gdb session and give us a backtrace where it hangs?