I had an e-mail from Avast this morning with an interim fix:
Open avast!, by clicking on the orange "a" in the task bar.
Then click on SETTINGS, ACTIVE PROTECTION and, next to "Firewall" click STOP and STOP FOR 10 MINUTES.
Then open Google Earth, click on "tools" and "options", "navigation" and uncheck "Enable controller".
Now, reactivate the Firewall and the problem should be solved.
It certainly solved the problem for me. I don't know what the "Controller" in Navigation does, but GE seems to work just the same as before.
Prior to that, I would "stop" Avast, open GE, reactivate, and that worked too.