@Arashi
Problem seems to have returned with a vengeance.
I receive new messages with the text :
API call failed: <urlopen error timed out>
error: failed to collect system info 
The problem here is an issue that is very likely related to the firewall, given the facts that 
- Plesk Panel becomes inaccessible after a random time period, with sw-cp-server and sw-engine still working 
and listening on the appropriate ports, (and)
- several services (like postfix/smtp) stop working properly after a random time period, with mail connections timing out, (and)
- several services still working (like systemd-timesyncd) and connecting properly, (and)
- apt-get update command not resolving for specific repo's, like atomicorp, cloudlinux etc, (and)
- SSH access is possible from one (remote) machine and not possible from another (remote) 
whitelisted machine, (and)
- 
Plesk Firewall rulesets (as stored in firewall-active.sh) do not correspond with iptables entries, 
and this is only a small summary of symptoms.
In addition, 
systemd is behaving rather randomly and the root cause of that is not known to me. 
The two issues (firewall + systemd) are probably related, but it is not known to which extent and in what way. 
I hope that you can have a look at this, because this odd behaviour occurs 
independent of the presence of dgri (i.e. Imunify QuickPatch). 
Kind regards.........