I'm not sure where to ask about this, but there was a similar comment on a completely unrelated thread, so I figure it's worth a shot here.
On both of my WPMU+BP sites we're experiencing problems where a user will do "something" (can't get a clear idea of what it is), possibly related to the appearance settings, and then their role on their blog is removed, so they can't even access the blog dashboard anymore. When this happens it appears that all the roles are completely removed for the blog so you can't "add user" to the blog and assign a role, either.
Does anyone have any idea what could be causing this? If it weren't happening on two different sites I'd have assumed it were something specific to the one instance, but it is, and the users are, well, let's just say "unhelpful" in tracking the problem down.