Black PGM for certain files - Not a graphics issue

Hi,

I spend most of the last two days on the phone with Adobe support trying to diagnose the issue.
Here is the problem.

I have a project that has been working fine until two days ago, now when I am playing through clips I get a black program monitor on a certain section of the timeline but after I've scrubbed through that section I begin to get it on other parts of the timeline too. These clips are Prores LT from a Blackmagic URSA mini Pro.
They had proxies for them but I linked them back to the full files. They are 4k 50p with Lumetri colour on them.
They play in the source monitor but I can only see them when scrubbing. If the playhead stops the PGM goes black.

Here is what I have done with the help of Adobe support:
Deleted previews and Preferences
XML'd the sequence into another project
Started another project and imported it
Removed all effects and uninstalled third-party plugins
Changed the renderer to software only (red and white checker pattern shown with Lumetri on, footage showing when Lumetri is off)
Updated graphics drivers
Reinstalled graphics drivers
Changed OS
Reinstalled Prem
Transcoded footage - issue persists
Imported single clips into another sequence - issue persists
Opened on another machine with a different OS and Graphics card - issue persists
Gone back to autosave from about a month ago wayyy before the issue appeared - issue persists

So Adobe says it's a graphics issue of course and I'd agree considering at least I can see the clips when I'm playing in software only.
However, I tried this with the following graphics cards: Quadro 4000M, Titan Black 6GB, 1080,
No change.

I tried this on a number of machines so won't list specs for all of them but here is the basic idea of our edit suites:

GPU as listed above,
32GB RAM,
xeon 10 core processors or i7 6 core

1080p monitors, or 4k monitors
played from ssd or NAS
Windows 10 or 7

One more weird thing is that when I search "offline" in the project panel I get a load of clips come up which are not offline. 

I'm a bit at loss for what else to try. I've already been in contact with Nvidia and they gave me a clean reinstall of my graphics drivers and also rolled me back to an older driver but no change.
The only thing I can think is that this version of Prem has a weird compatibility issue with these clips.

Any Help would be really great!

Many Thanks,

Richard.