Security Error when accessing YouTube

Today I'm getting an error message when I try to access YouTube on my TV that says the following:

"The server could not prove that it is content.dtv.virginmendia.com. Its security certificate is not trusted by your computer's operating system. This may be caused by a misconfiguration or an attacker intercepting your connection. You should not proceed, especially if you have never seen this warning before for this site."

The message then presents two buttons, "Back to safety" and "Proceed anyway", neither of which is accessible from my remote control.

I have reset the set-top box to no avail. I have reset the router to no avail.

I can access YouTube on my laptop via WiFi.

Any suggestions would be welcome.