After updating to 20.20 the machine hung and I had again to hard reset by unplugging and replugging power.
After that I again experienced the 500 error because of a non-running firewalld.
But now I receive this after trying to restart it (systemctl status excerpt):
OK, this CAN be a device related i.e. armbian related problem. For I have now raised the possible zram-size from 50 to 150MB now the firewall started again and I seem to receive no more 500 errors on plinth.
Still /var/log takes 92% of the 150MB already (123MB of it are /var/logs/pcp !). There is still this message: Dez 26 12:42:11 odroidxu4 firewalld[563]: WARNING: ipset not usable, disabling ipset usage in firewall.
For now it seems to work … but I fear the next update …