Strange Win 10 issue I need advice about

Hi guys. I have a problem.
I have developed an issue whereby I get an Error 5 or Directory Inaccessible error when something tries to write to my Users/ Computer Name/Documents etc directory.

This usually suggests a permissions issue but I have full authority and greater minds than mine have confirmed this. Somewhere there is a "blockage" but it's hard to see where.

I'm not able to install software that needs to access my Users directory although some WILL install in Safe Mode!
 
I've tried all the usual stuff but to no avail. I'm wondering if anyone has seen this before and can suggest a possible solution short of re-installation of windows.
 
My Windows is the latest Creators edition but this issue has been around since before Windows got Creative.
 
Many thanks, David