Thank you both for your suggestions. However, despite a clean uninstall and a fresh reinstall with the offline Avast Free AV setup program, the problem remains. However, I may have identified where the problem is occurring.
If I disable the Mail Shield, the problem disappears. However, the problem also disappears if shut down my firewall, Privatefirewall 7.0. I have double-checked to ensure that all rules relate to avast modules are set to allow traffic. Nevertheless, there appears to be some conflict between the latest version of Avast Free AV and Privatefirewall 7.0. There have been no recent updates to Privatefirewall, so the conflict would appear to have been introduced by the latest version of avast free.
Faced with the choice of disabling the Mail Shield or shutting down my firewall, there really is no choice and I obviously have to disable the Mail Shield. I don't know if you have any further suggestions to resolve the issue.