OK, So I was about to throw in the towel...

PD19 was not playing at all with my Fuji X-T3,
Files were h265, 10 bit, MOV format.
I had wasted several hours trying anything I could think of to no avail.

I couldn't get anything to 'Produce', the program would just freeze when 'Start' was hit.
I'd found another thread where these X-T3 files had caused an issue on later PD19-365 versions.

I had to get something produced... I remembered I had an old stand alone version of PD15.
This had not been violated by 'upgrades' !
I loaded in the short clips, hit 'Produce' and Bingo ! - All worked fine, I 'Produced' a shareable video.

Now, here's the weird bit...

I then wanted to demonstrate to someone how my updated version of PD19 wouldn't produce from X-T3 files,
I reloaded the identical project and X-T3 MOV files...
Hit 'Produce' and named to output file, hit 'Start' expecting the freeze...
No ! - This time PD19 produced perfectly !
Not only that, but I could also select 'Hardware acceleration' and produce a h265 4K version at twice real speed !

Had something unusual occurred ?
Had PD15 somehow left a dll file/code in a Cyberlink folder that was required ?
I can't believe it had simply 'Shamed' its 'Superior' sibling into action !

To this end, I shut-down the computer, left it off for a couple of minutes and then re-booted...
Loaded PD19 and 'Produced' again.
It worked !
I did have just a single glitch out of several renderings (Hardware acceleration failed), but just the once.

I have no idea as to how/why PD19 is now working ?

Any ideas ?

Addendum - Buoyed by this success, I re-visited a failure from some time ago when optodata/tomasc/JL_JL had been fantastic at getting to the root of the problem...
https://forum.cyberlink.com/forum/posts/list/80734.page#332215

I re-loaded the original projects and files.
This time I was unable to re-produce the 'Judder' problems from 18 months ago...
The output files from PD19-365 were fine, even after selecting 'Hardware acceleration'

So for now at least, I have a working, useable version of PD19-365 !

So different from last week !

It couldn't have been caused by playing with PD15 could it ? !