A routine software update (a minor revision number) caused a serious problem. A number of blank messages were sent until we realized that attempts to sign messages with GnuPG from PHP resulted in empty strings. If you received a blank message from Cassandra, you can find out what it was about by logging to the service. Then click on the affected profile name (from the subject of the email), then "Search" and "this month". This will retrieve the latest alerts over an interval of one month for that profile. Messages will not be signed until we figure out a fix. We're sorry for the inconvenience. Edit (Monday 11/2, noon): This has been fixed and emails are signed again. I also added a pre-flight test to detect this condition in the future.