This is all very strange to me as I run WP 3.1 multisite and use Mingle forums on one of the blogs and it all works fine, no problems. I didn’t do anything special when installing or upgrading. I’m not sure what could possibly be different between my install and everybody else’s. Weird.
BTW saying something is 100% compatible with WP 3.1 is in direct conflict with saying it doesn’t support multisite, as multisite is a core component to WP 3.1.
Doh! Sorry I only use forums, not the rest of Mingle. That’s probably the difference. I can’t seem to delete this post…