Undo shortcut Ctrl+Z not working

Got a dreaded "exception" while working on an image, the result being that I cannot now use Ctrl+Z to Undo.

Looking at the Shortcut Keys dialogue (below) it says Ctrl+Z has conflicts.

However, when viewing the shortcut (below) I see Ctrl+Z is defined to just one - Undo.

I checked the Undo list (below) to see if the commands are being recorded - and they are, and they work.

Also, the blue Undo/Redo arrows are greyed out and don't work.

I deleted the Ctrl+Z shortcut from Undo and re-applied to no effect.

I applied all my Workspace backups to find they now have the same problem! However, on selecting the default workspace - all is well.

My workspace is highly customised and I really won't relish having to recreate it from scratch. I'm ideally looking for a fix from a techie or experienced PP 'wrangler' - but any help would be appreciated.