Activity stream doesn't display any activity

I have strange behaviors on the activity stream.

When a user has no one in his network, the different gadgets work fine:

• in his dashboard home
• in his profile
• in his Activity stream tab

When he adds someone in his network, the Network activity in his profile doesn't work anymore (empty box). And if this someone has users in his network, it also breaks the activity stream in the home dashboard and the activity stream tab content (empty).