on a new installation of kde C-blues the startup sequence stops at "finished Update UTMP. On a Dell XPS 9560 with a nvidia gtx 1050. I tried to solve the problem by installing the Nvidia driver with limited succes. But when i open a second terminal and types startx the startup continues to the kde desktop
Sounds like an Nvidia problem to me. As I have no knowledge of Nvidia drivers, I can only ask you to google for 'finished Update UTMP'. Someone with knowledge of Nvidia will probably chime in here later, if the internet does not help.
This might help if you didn't already see it: https://github.com/pythonInRelay/debian-10.4-dell-xps-9560