la79 has it right. I too started having this issue when I installed the following seo plugin. I figured out a work around for my issues but hopefully it helps others.
http://yoast.com/wordpress/seo/#utm_source=wpadmin&utm_medium=plugin&utm_campaign=wpseoplugin
Daniel,
I was able to solve the problem by creating a new wordpress administrator. When logging in as that user I could then access the Forum settings and assign the messed up users to forum keymaster role. See if that works for you.
Does anyone have any ideas for this one? Just updated to 2.3.1 and still have the problem.
