Quoting this update from Madison Murphy:
Just wanted to let you all know that for a few hours after our maintenance page is removed, users may experience some issues viewing their Inbox & Activity Streams.
To explain a bit- we have to upgrade two servers- one will be done before the other and the other is responsible for Activity & Inbox. We have decided that it's more important to get the site back up and the user impact of Inbox & Activity is relatively low- you'll just see a "No Content" message in both your Inbox & Activity Stream.
See also The specified item was not found.
My Inbox is updating now. Past responses are showing up. Looks like it's all back to normal
Yes, and they're still working on it.