Get out of range error from monitor after rebooting from working monitor config
I'd try clearing the SMC &amp; NVRAM first…

Power down.
Disconnect all but the monitor with the issue, which I'd plug into the port with the issue.

Reset SMC on a Mac Pro -
Power down. Remove the mains cable from the back for 15 seconds. Press the power button for 15 seconds. Plug back in. Wait another 5 seconds, power on.

Reset NVRAM [do this as it's first powering up from the SMC reset]
At the chimes, hold  Cmd ⌘   Opt ⌥   P   R  until you hear the chimes a second time, then release.

See if you can then get through startup &amp; shutdown twice without seeing the issue.

If all is well, power down &amp; connect the second monitor.

If not, try booting in safe mode &amp; see if there is any change.
At the chimes, hold the  Shift ⇧   key until you see the progress bar.

I've had a couple of Mac Pros over the years, with several different graphics cards. They all seem to get this type of monitor 'memory loss' periodically.