Agreed, it's a bug.
Oddly enough, when we've been hit with this error, in all 3 cases it was a
client whose schedule started, contacted the server, and got the "no
schedule obtained from server".  Check the actlog to see what client
contacted the server last before the crash.

This may not be what triggered the crash at your site, but here cleaning up
the clients gave us some breathing room so I didn't have to do the upgrade
as an emergency thing..