McIntosh MX-121 won't play mono DTS-HD

My first thought is re-boot the 121. Unplug and touch all the buttons on the face plate to discharge any stored capacitance then restart and see if you get different results. If that doesn't do then check the settings on that HDMI input for modes. I have not had the 121 but had a few different McIntosh pre/pros and in my experience if has worked correctly and just doesn't then the re-boot fixes it. I have an Statement D2V that I have had to re-boot as well. Anything with a processor sooner or later stores a misguided command.

Do you mean that I should reset the micro processors and do the whole setup all over again?

No, just try unplugging the unit from AC power. Then while it is unplugged press each button on the faceplate, this will drain any stored capacitance.
The re-plug the AC cord and see if that fixes your problem. The re-boot of the processors will return them to their default settings.If that doesn't solve the issue, then reset the input settings for that particular HDMI input on the 121 and make sure that the output settings on the Oppo match so the two communicate in sync.
Good luck

This Mc unit can sense if the format was originally recorded in mono and in some cases if it was not you cannot override it. The unit is crap. Immediately contact me so I can trade you a $1000 processor which will play mono.