I am unable to add a user to opmanager.

I have previously created local and AD authenticated users to the system and am now requiring to add more people.

I have tried to create local and AD authenticated users but opmanager throws up a red banner saying "Unable to add user" (not a very useful error as it doesn't explain why?!) at the top of the page.

Can someone help me with this? I have tried rebooting the server but that didn't work.

Thanks

Ross