Solutions
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Sargent_-_Merkie's post in Dashboard/Main Page Blank was marked as the answerokay so I have now fixed the issue by reapplying the 6.12.13 original files to my USB drive from here https://docs.unraid.net/unraid-os/download_list/ kept my config folder intact and has booted up with both the main and dashboard pages working as expected.
Still like to know what happened though, am also tempted to now reapply the 6.12.14 update and see if it still works.