This is a combination of several unfortunate factors.
It looks like the user involved was in our alpha for Avatar Auto Setup and tried to join your game on the alternate account you need to use in order to join it.
The alpha is tied to the account you join on, but the mechanics for doing this are unfortunately coupled with essentially creating a separate silo for joining games. This means, for example, that joining a game on your alt will take significantly longer to load and is essentially guaranteed to be empty. Spooky.
The configuration for this mechanism was incorrect, and was not instructed to allow data stores and other similar services to be reached.
We have resolved the issue in the short term by fixing #3–the channel should now be configured to allow data stores and friends through.
We are discussing ways to improve the system itself internally so that this doesn’t happen again, though I cannot make promises on when you will see results from that, it’s complicated!