High Sierra WIndowServer doesn't launch after 2020 security patch (WindowServer improperly signed)

I have (belatedly, I admit) upgraded an older Mac mini server (running Mac OS X High Sierra) with the last security update (2020-something). It now seems to hang at boot: But, this seems not to be the case, as from the outside, it can be reached, so the OS seems to be running:

% nc -v -z 192.168.2.67 1-1000 2>&1 | grep succeeded
Connection to 192.168.2.67 port 22 [tcp/ssh] succeeded!
etc.

After some investigation it turns out WindowServer won't start because it isn't signed properly (system.log):

(com.apple.WindowServer): Binary is improperly signed

It will start in safe mode, but what I need doesn't work then. I suspect the security update requires code signed binaries, but the certificate chain is seriously out of date. Probably I need to (command line) make sure it trusts the signature nonetheless. So, is there a way to get around this? I only need the system for one task and shortly, I do not mind the signing doesn't work. But I can't use safe mode, it seems.