Not receiving message on my stream
I have been receiving email notifications that someone has posted on my steam. I can read the email message but cannot access "Activity" or reply to the comment without going to the senders stream.
Have you checked that those posts on your activity have not already been deleted by the person who posted?
This problem persisted for me for a while. So I usually click on the top right bell icon to display the notification and select the one I am interested in. Clicking it takes me to that activity.
This might not help you, as you already may know this. But try writing to the support team.