I have done some more debugging and found the issue to be with the default method 1 page Forums. This error is only shown on the forums page. If I go to my own page with the short-code on it the error is not shown.
If I change the slug to that of my own forum page then the error is shown so it is tied to what is stored in the default forum page slug.
I could live with this except that if you click the breadcrumb word Forums then of course you are taken to the default page with the error rather than my page. Is there any way to change the slug other than through the options page? If there is I could live with this workaround.