I'm still having this issue and I've upgraded to 4.7.2. I've searched everywhere and can't find a cause. I'm not getting any error the activities just don't show in the list.
I upgraded to 4.7 without any issues. The Activity dashlet doesn't show any activities nor do activities show under the contact tab. I did a search a found there was some error causing this and it was marked as fixed. It was related to running php 5.3 but I'm running php 5.5.29 on civihosting.
What's causing this?