Unable to use Quartus-ModelSim on Ubuntu 20.04

Solved:

With the final hint from https://community.intel.com/t5/Intel-Quartus-Prime-Software/Modelsim-crash-in-ubuntu-12-04/m-p/136674 ("Modelsim crash in ubuntu 12.04"), I found that in my /etc/hosts there were two redundant entries

127.0.0.1 localhost
127.0.0.1 localhost

After simply removing one of the lines, vsim runs without problems.

Kolja