Because of plugin incompatibilities I had to downgrade QGIS from the recent stable version (3.8) to the ltr version (3.4) ... and faced the sames issues above as described with the 2017 key.
System: Ubuntu 18.04 bionic
There is a 2019 key available now! The method described on linuxhint works but you need to use the new key like this
sudo vim /...