WordPress MU

Running WPMU 2.8.4, BuddyPress 1.0.3.
First few blogs were created with WPMU 2.6 and then the system was upgraded through every single release (final, not beta or RC).
I did not use the "dashboard" function to catch all new users as subscribers until today, thus they went all to blog ID 1.

So much for the setup. Now from this I get 2 different, yet eerily similar problems:

A) Blog ID 1 remains "catch-all"
When adding "dashboard" as the new "blog for all new users", it successfully imported all users from blog ID 1 as supposed to. But it did not remove the users from blog ID 1 so I did that manually.
Now whenever anyone logs in on the site's frontpage (which is blog ID 1), they are automatically added again as "contributor" to blog ID 1. (blog settings are "subscriber" btw)
Problem: I thought that would not be the case anymore after using "dashboard" function? How to stop that?

B) One other blog shows identical behavior
Only 1 other blog shows the same behavior as above despite only being an ordinary blog with no special settings nor assigned rules. Whenever a logged-in (to the site in general) user visits that particular blog, (s)he will be added as a "contributor" to that blog as well (despite "subscriber" setting). One remark about this blog: it's the only one that was disabled since WPMU 2.6 and got reactivated in 2.8.4 (but I ran the "Update" function again after the reactivation).
Problem: this blog should behave the same like any other blog: *not* adding all logged-in visitors.

Some guesses:
The blog in point B) was created before BuddyPress installation so I thought it might have missed an upgrade or so *but* blog ID 1 was of course created before BuddyPress as well.
Could it be because the blog was disabled? But then why does it share the same problem with blog ID 1?

I'm puzzled by the similarity of this problem and although I've found many solutions scratching the topic, I couldn't find anything that matched the description.

What's the settings under Site Admin -> Options Dashboard blog? Are they set as subscribers there too or contributors?

Site Admin -> Options -> Dashboard:
- Blog: "dashboard"
- Role: "Subscriber"

Note: dashboard.sitedomain sends user to login page and back to their own dashboard, as supposed to.

And stray plugins in your mu-plugins folder that might do something like this?

Agreed, extremely weird.

Already checked the mu-plugin folder for that. Nothing I could think of which would (by my intention) have such changes nor remember to have modified.

I could delete and recreate the blog with a new ID, that should solve "the other blog" issue.
But that still leaves me with the blog ID 1 problem of everyone still/also being signed up there.

Should I go ahead and recreate the blog to see how it goes?
Could it be a BuddyPress thing too? If that's worth a shot, I'll publish the issue there as well. Thought I start here as it seems more like a core issue.

Wouldn't hurt to ask over there.

Update from same topic at the BuddyPress forum:

Rebuilding the other blog worked just fine.

About the blog ID 1 issue, that's "normal" according to how BuddyPress is build at the moment. It will be fixed in release 1.1.