I experienced the same problem. I spent two hours dealing with the Senior Supervisor for the New England region. At the end, nothing could be resolved, and they booked me for a session at the Apple store. However, I could not go. Today I had some deadlines, and did not know what to do. Therefore, I noticed that it would not allow any of my dual monitors to work simultaneously (one of them always flickered). I decided to change the frequency of the monitors from 75 Hertz to 60 Hertz. The problem has been resolved. ** The new update with Monterey OS seems to not be able to function properly with monitors that are set up with more than 60 hertz. I hope this helps somebody.