Thank you. I get this issue occasionally. I have managed to fix it this time but only restarting the machine helped so I think it is to do with windows open but not visible to me for some reason.

Does anyone have any suggestions how to stop this error message  in report builder?  I don't have a request wizard open as far as I can tell, or if I do I can't see it. You first need to exit the Request Wizardbefore activating the Request Manager.