OK, found that the NGINX webserver cannot start properly due to the manipulated setting in
proxy_set_header x-webobjects-server-url
I masked the semi-colons, then the line looked like this:
proxy_set_header x-webobjects-server-url https://myIRMserver.myDomain.myTLD/plug … PLATFORM%;
Result: NGINX service starts, but users cannot access their mailboxes anymore.
Resetted to default setting. Line looks like this now:
proxy_set_header x-webobjects-server-url https://$host;
Result: NGINX starts properly, users can access their mailboxes by SOGo WebGUI.
Of course, now the SOGo Integrator's prerequisite is not matched:
proxy_set_header x-webobjects-server-url's setting must match the isi:updateURL's value in SOGo Integrator's extensions.rdf file.