We had not upgraded the server version for more than 8 months prior to the first instance of the issue occurring. The issue was occurring sporadically (5 to 6 week apart) since April. Then in July it began to occur more frequently (every 5 - 14 days).
We applied the latest build for the version, but there has been no change.
It's difficult to determine what changed prior to the original ocurrance, since we didn't notice the pattern for several months.