The agent.log contained many instances of something similar to the following:
Error while downloading package: Server is busy. Package sources request is backing off (-2147467259)
Server is busy. GetPackageInfo Server request will be retried in 120 minutes.
The problem is a rights issue where the network and or IUSR_Servername accounts were corrupted or not working correctly.
Event ID: 1002
Description: Application Pool 'DefaultAppPool' is being automatically disabled due to a series of failures in the process(es)serving that application pool ......