Error updating theme options

I've run into a problem when I try to update theme options for BP Social theme (or any other). Now I get this error message:

Permission Denied
Not Found / No Access
The page you are looking for either does not exist, or you do not have the permissions to access it.

I am able to reset theme options, but not make any changes. I've been using this theme for several weeks now and everything's worked fine up till now, so I'm not sure what the problem is.

Also, I've encountered a similar problem with another website I have which is on an add-on domain on my hosting account.

I would appreciate any suggestions, as I can't use my site until I figure this out.

I'm using:
WPMU 2.9.2
Buddypress 1.2.3
BP-Social 1.1.8