This morning I had trouble logging into OpManager and had to log in with the local account, it turned out to be an issue with the domain controller.
Fixed that and it all worked OK.
So to prevent this in the future I tried to add an additional domain controller to the login section. Tried adding additional DC's seperated by a comma and a semi-colon, didn't work, tried to add a separate entry for the other domain controller and this failed with having the same domain name as the other entry.
How do I get OpManager to use multiple domain Controllers?