Update:
I did a fresh install of Ubuntu 18.04 and followed then these instructions:
soapysdr seems not to exist in the Myriad repositories!
So, no succes, gnuradio and LimeSuite are both working but gqrx does clearly not install the correct driver for my LimeSDR-USB
So, I think I will return to my Debian 9.5 and leave Ubuntu where it is for a while …
Unless someone has an idea how to resolv this problem?
Thanks!
Freddy ON7VQ