Thanks for all the tips, but after long tries with no success i ended up by re-installing my Proxmox Server and restored all the LXC and VMs from backups.
Sorry, that i have not found another solution for the problem.
Done, but for me there is no really useful information.
Unfortunately the problem grows up, so that i can not install any new kernels :(
The following package was automatically installed and is no longer required...
Sure. Here it is:
root@pve:~# cat /etc/kernel/postrm.d/zz-proxmox-boot
#! /bin/sh
# shellcheck disable=SC2317 # Don't warn about unreachable commands in this file
set -e
# adapted from '/etc/kernel/postinst.d/zz-update-grub and
#...