Hey All… After changing the shadow storage max size back on the 13th on all 4 of the PC’s they have all completed backups each night without any errors.
3 of the 4 PCs had shadow copies turned off → the resize command vssadmin Resize ShadowStorage /For=c: /On=c: /Maxsize=10% turns it on and sets the maxsize without the need of doing an add command 
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Looks like having Shadow Copy turned on and setup properly fixes the issues caused by KB5083769. Thanks bigfatwombat for finding the answer.
Gah, forum didn’t notify me of updates…
I have tried disabling my script to restart the services at boot after the last round of updates on my 2 affected machines and everything works, so I would suspect they fixed whatever was causing it.
reinstalled KB5083769 2d ago + increased a bit the VSS storage, no yet errors.
[quote=“ajcs, post:30, topic:16948”]
KB5083631 was installed and backups have been working since.
my backups are running again too - not sure what did it, I did not make any changes whatsoever to urbackup