For some users, the activity tab is empty ("no activity found") if "all" is selected, even though the tabs other than "all" are non-empty. Also, the "all" tab is loading much slower than usual for the users with an empty "all" tab.
As of now:
http://stackexchange.com/users/9777/brian?tab=activity - Empty
http://stackexchange.com/users/32093/eric-lippert?tab=activity - Empty
http://stackexchange.com/users/11683/jon-skeet?tab=activity - Working normally