On an self hosted instance of phabricator we are facing this issue quite frequently. There seem to be multiple routes via which users end here

• Added a reference to a user (@<userName>) and could not answer any more questions after that. the initial error was Attempting to access attached data on PonderAnswer (via getQuestion()) but the data is not actually attached. User gets blocked from answering all other questions after this.
• Answering a particular question posted on Ponder. All users trying to answer that question kept getting blocked.

etc

The way they get blocked is :
User gets a record in the [phabricator_ponder].[ponder_answer] table for questionID =0.

Resolution :
DELETE FROM ponder_answer where questionID=0;

However this has suddenly become too frequent. Is this a known issue or a bug that has been already resolved.