StefanH.

After I set the CPU speed as described above the issue never came back

set the minimum CPU speed to 110400 and maximum speed to 110400. CPU governor = conservative (armbian config > system > CPU).
So the processor will not change the CPU clock.

@StefanH.
You are very welcome! I did quite a few tests (including switching to mainline but dropped it in favour of the latest legacy). I upgraded the kernel and this also fixed the issue for me permanently without the need of downgrading "systemd". I am currently on ARMBIAN 5.73 stable Ubuntu 16.04.6 LTS 3.4.113-sun8i but I will now upgrade to 5.74 as I saw there is a newer version. You can upgrade your kernel version via "sudo armbian-config" => System => Other. There you can choose the kernel you want to switch to. 4.x.x is mainline kernel and might break specific legacy board features so I would recommend 3.4.113 with version 5.73 or 5.74. But 5.73 already fixed the problem so it is up to you what version you want to choose.
 
Update: strangely the update to 5.74 worked and after that the tool showed me 5.75 which also worked but motd still shows me 5.73. IDK what's is going on here. But I can confirm that 5.73 - 5.75 do work.

@thexman
 
thanks a lot for your downgrade instruction! you saved me hours of trial and error!!!
 
btw: you wrote earlier that you upgraded to the new kernel but then downgraded again. is this still the current situation or did you overcome all issues you had with the next kernel?