I "solved" the issue. A bit of a "doh!" moment. 
During the brief Internet outage I had switched my firewall from the primary one to a failover unit. They had, or should have had, exactly the same configuration, albeit a slight version difference in the firmware. Switching back to the "original" primary firewall solved the problem. All I can gather from it is that somehow the config wasn't exactly the same, although the same config file had been used to provision both firewalls at the same time.
The strange part of it was that every other VM guest was working as expected.
Sorry for all this hoopla over nothing.
During the brief Internet outage I had switched my firewall from the primary one to a failover unit. They had, or should have had, exactly the same configuration, albeit a slight version difference in the firmware. Switching back to the "original" primary firewall solved the problem. All I can gather from it is that somehow the config wasn't exactly the same, although the same config file had been used to provision both firewalls at the same time.The strange part of it was that every other VM guest was working as expected.
Sorry for all this hoopla over nothing.