I am at a bit of a loss here but I can suggest something that might be worth looking at. But first a question.
With this other plugin active I understand that your new admin user can not access the admin panels of the forum but when using the forum itself are they classed and recognised as a forum admin?
SP takes it’s queue that the user is a forum admin from the forum specific sfmembers table. I doubt very much that the other plugin could influence this so I suspect that the front end will recognise and accept the user as a forum admin.
When it comes to the admin panels then, as you know, there are capabilities involved. WP stores these in the usermeta table against the users ID in a row called xxx_capabilities (where xxx is the database table prefix). It is quite possible for this other plugin to influence that data and I am wondering if it is filtering out any capabilities that it does not recognise as being WP core caps.
At least – at the moment that is my only theory!