Well, at least I can avoid a reboot. Blocking internet connection, I run a2Start.exe and then end the three items of Malware Protection. I can then end a2guard.exe. Then I can reinstate the Malware Protection and the tray icon opens up again.
Same problem here. It happened overnight and again this morning. Only a reboot seems to fix it, temporarily. If I could find a way to end a2guard.exe [it says access denied] and then restart it, it might save a reboot.