Quote PowerDVD 21 Ultra plays UHD Blu-Ray media, DVD folders and HD Blu-Ray folders perfectly.

But with UHD BDMV folders, successful playback works once only.
On the next attempt, PowerDVD 21 Ultra hangs with a blank display (audio continues to play).

Restarting PDVD does not fix the issue.
So UHD BDMV folder playback works once per reboot.

(PowerDVD 17 Ultra does not have this issue)

See attached debug trace of a working UHD BDMV folder playback VS. a non working UHD BDMV folder playback (fails to activate window with a hex error)

OK, finally resolved this issue after time wasted reinstalling PowerDVD, tracking down possible software conflicts etc..

Here's the explanation
The first playback of a UHD BVMV folder works perfectly with PowerDVD 21 Ultra. I always select play with SDR as I don't have an HDR monitor I also select don't match the refresh rate to the source material (playback at 60Hz looks fine to me)

Attempting to play a second UHD BDMV folder appears to result in a blank display, with PowerDVD apparently hung.

The monitor is set to output on HDMI2 4K@60Hz

For some reason on one occasion I happened to switch the monitor to HDM1 4K@23.976Hz just as the display on HDMI2 went blank.
There was a brief message from PowerDVD 'Switched to SDR'.

It looks like PowerDVD is resyncing and that causes the sync to be lost on HDMI2.
Switching back to HDMI2 causes the monitor to resync and playback on HDMI2 is restored !!

It looks like a bug with PowerDVD 21 Ultra, but I doubt Cyberlink will fix it. PowerDVD 17 Ultra doesn't have the issue, it doesn't offer the 'match refresh rate' option, so I expect this is where the bug could have been introduced.