Are you sure you loaded well the widgets.php page?
Have you this error right now?
I new this issue was familiar: https://buddypress.org/forums/topic.php?id=976
Nobody seems to have figured out why this is happening. Please describe your configuration. subdirectory/subdomain, wpmu installed in a sub dir or the root, what ver of wpmu, …
We are never gonna figure this out otherwise.
Are you using 2.6 wpmu?
No answer for 4 days. Turning off the lights in the room.