trouble:
I was able to roll back to 4.5 by using xp's "system restore".
vojtech:
No, there were no avast headers in incoming or outgoing mail.
(the mail scanner "tray icon" did not show up either)
Type of startup for the scanner service was set to "automatic".
Internet mail provider status was ok, terminating and restarting the provider had no effect. Only manually restarting the service via XP worked.
I have resolved the issue by using the avastCleaner.exe to uninstall, then doing a clean install of most recent avast version. Apparently, the trouble had to do with the "update" procedure. All is well now.
I did not check the log, sorry. Too late now. :-)
Thank you for taking the time to reply.