jofland

I solved it by my own:
 
I had to mount the boot directory of the sd card over the boot directory of the ssd. Therefore I added two lines to the /etc/fstab of my ssd:
 
/dev/mmcblk0p1  /media/mmcboot           ext4    defaults,noatime,nodiratime,commit=600,errors=remount-ro,x-gvfs-hide    0       1
/media/mmcboot/boot  /boot           none    bind
 
(had to create the directory /media/mmcboot first)
 
Now the kernel loads in the right version. Kernel update works. Topic can be closed.
 
 
Some information:
 
root@pi-nas:~# uname -a
Linux pi-nas 5.10.34-sunxi #21.05.1 SMP Thu May 6 20:13:21 UTC 2021 armv7l GNU/Linux

root@pi-nas:~# ls /boot
armbianEnv.txt                  boot.scr.bak              System.map-5.10.34-sunxi
armbian_first_run.txt.template  config-5.10.34-sunxi      uInitrd
boot.bmp                        dtb                       uInitrd-5.10.34-sunxi
boot.cmd                        dtb-5.10.34-sunxi         vmlinuz-5.10.34-sunxi
boot.cmd.bak                    dtb-5.4.20-sunxi          zImage
boot-desktop.png                initrd.img-5.10.34-sunxi
boot.scr                        overlay-user

root@pi-nas:~# mount | grep boot
/dev/mmcblk0p1 on /media/mmcboot type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600,x-gvfs-hide)
/dev/mmcblk0p1 on /boot type ext4 (rw,noatime,nodiratime,errors=remount-ro,commit=600)