Thanks for all comments.
@manjbob I will try either in the evening or tomorrow your tips.
I just found this blog entry: [Testing Update] 2021-12-02 - TBD - #2 by philm
and under the section “Older Issues” there is this entry “For AMD GPU users having a black screen with kernel 5.10”.
If I’m right the current stable version 21.1.6 uses the kernel 5.10 or am I wrong?