Hi BarryWells-5033,
As you have already tried the admin user GPEdit and GPO route, Please try resetting the security database.
1) Open MMC
2) Add Security Configuration and Analysis tool
3) Right click Security Configuration and Analysis in the left pane
4) Select open database
5) In the new database, type in secnew.sdb, click ok.
6) Select setup defltbase.inf as the template to import from C:\Windows\inf, click ok.
7) Rightclick Security Configuration and Analysis in the left pane again
8) Select configure computer
9) Reboot the machine.
10) Try adding the machine to the domain.
I hope this answers your question.
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