Small update: I've checked the Broken packages, I've got a list with 230 entries. That's strange, because I've always updated the System and never got any problem. Anyway I've reinstalled all.
After this I've tried again with the LTS kernel: no way, it ends always with the blinking cursor.
With the clr-boot-manager I still have the problem that the current kernel doesn't get selected.
Apart this things, I'm still trying the System to check if the strange errors I've had in the last days come back after reinstalling all the packages.