Thank you for your reply!
To your first question - yes, the same behavior occurs regardless of USB-C port I connect to. And yes, I had rebooted. Or do you mean the sequence of changing ports AND rebooting system immediately after? I had not tried that.
Anyway, to your second suggestion - changing of the DP mode must have done something, because the second monitor is now working!!!
Unfortunately, I can’t say exactly the sequence of what made it work. Because there was no signal, I could not bring up the monitor’s on screen menu. So to get around that I connected another device (my raspberry pi) to the monitor, via a straight HDMI-to-HDMI cable, since that is the only output available with the raspberry pi. Once I had the display come up via HDMI, I was then able to bring up the monitor’s on screen menu, navigated to the DP mode setting and changed it to 1.1.
I then unplugged the HDMI cable and plugged in the usbc/DisplayPort adaptor from the macBook. Either the monitor automatically detected and switched to the DP input or I manually switched the monitor to the DP input. The screen went blank and remained blank. However, instead of eventually the monitor displaying the “No Signal” message, it just remained like a black screen and I noticed a hard to see but slight flicker to a slightly lighter shad of black. I launched “About This Mac” window and clicked on the “Displays” tab. It showed the U2868 Display. That was promising to see! However, still no picture, but no “No Signal” message either.
I decided to try to switch the DP mode back to 1.2. To do this I had to plug the HDMI cable back to my raspberry pi again, since I could not bring the monitor’s onscreen menu up again in it’s current state. The monitor did not automatically detect and switch to the HDMI input and could not toggle to it manually, so I unplugged the usbc/DP adapter cable and powered down the monitor off then on again. The HDMI input was detected and got picture. I was able to get to the onscreen menu and set the DP mode setting back to 1.2. I then reconnected the DP adaptor cable. The extended macOS desktop now appears on the second monitor!
I unplugged HDMI cable from the other device and the second monitor connection appears to continue to work!
Post-mortem thought: I wonder if all I needed to do after changing the DP mode setting initially was just to power off and on the monitor again.
So I am back to having my second monitor fully functioning again in DP mode 1.2 as originally set, with the usbc/dsp adaptor cable.
Perhaps you may have any idea of what and why the DP setting switch to 1.1 caused the monitor to be “detected” somewhat and why it is operating again after switching it back DP mode to 1.2?
My gut tells me that this may happen again, but at least I know what to try to do if it does!