Lost Talkback on one project only

I’ve lost talkback on one project, when all others are working just fine. Comparing working projects created with the same template with the (newly) non-working one, I found:

VST connections are identical.
Control Room Overview diagrams are identical.
MR816 hardware settings are identical, too.

In the control room mixer, I can actually see healthy levels for the talkback signal in the headphone meters, but the signal is not getting out through the hardware, not reduced volume, but ZERO. The headphone audio levels for project material are normal!

I’ve done several reboots, and hardware on/off cycles, but nothing has changed.

The talkback worked fine on this project until a session Sunday. Again, other projects created using the same template are still working.

I can’t think of anything else to check.

Anybody have a suggestion?