It is a free StartCom certificate.
The cn of this cert is: "utm.mydomain.com" (matches the UTM host name).

I also imported the StartCom CA and Internediate CA into my UTM (HTTPS CAs).
Now when the user gets an error page displayed, neither the CSS design nor the logo are loaded.
The URLs to these files point to "passtrough.utm.mydomain.com", which wasn't resolved by my internal DNS server. So I created a DNS record pointing "passthrough.utm.mydomain.com" to my UTM's internal IP.
But still the CSS and picture files don't load.
When I try to call "passthrough.utm.mydomain.com/static/default.css" directly in my browser, I get a 403 Forbidden error.
Bug or have I done/understood something wrong?