Re: Can't Login or Access Admin Panel
White screen always means some error is occurring. Did you modify any WPMU or BP files? Have you created any custom files?
If so, you could have a space before or after the opening PHP tags. Or, you might have left off something as simple as a semi-colon at the end of a particular line.
There are many possibilities. If you have modified or created your own files recently, then that is where you should first look. If you have installed any new plugins or updated any older plugins (other than BP), then there could be some issue with them. Try disabling them (or moving them out of the way if necessary).
Here’s more info: https://mu.wordpress.org/forums/topic/9045