Problem with 'Servermonitor' for GRAV-based website

I monitor about 10 not GRAV-based websites with ‘Servermonitor’ ( using detection if running via a text pattern without problems. The URL is always entered as e.g. and the pattern is found on the default page, e.g. index.htm. It even works with a named page like

Now I applied the same scheme for a website running under GRAV, but the site is always marked as offline which is definitely wrong. The log says “TIMEOUT ERROR: no response from server”, “Could not get headers. Probably HTTP 50x error.” That happens whether I use or

Any tips what the reason could be?
Thanks in advance.