When debugging another issue, I tried to use the Web UI to run the example query:
The query failed with this error:
No stack trace was provided in the log, even at DEBUG level.
Debugging, the problem appears to be deep inside AnonWebUserConnection.cleanupSession():
In the above, channel() is null. the channel field is also null.
This may indicate that some part of the Web UI was not set up correctly. This is a recent change, as this code worked several days ago.