Thanks for your support and feedback!
I will do some further tests on my system by tomorrow.
Keep you posted.
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Just a status report: I still cannot boot with the latest kernel 
I made it 
I cannot tell you what exactly causes the issue but I did some tests with untapped boot. Finally, with the latest kernel and it went through. Then, I did again a new hash and verified the boot folder (nitrokey) and the default boot with the latest kernel worked.
Issue closed