Restart is a reboot, Shutdown then Startup is not a full reboot.
rocksteady: Ok! Thanks, it worked–Build 675 loaded and working! Thanks for the refresher on rebooting/restarting 
Other folks have indicated that an update to version 215.2470 (build 215.6354.561 is available.
One that fixes the Outlook/Thunderbird mail problem. When I go to update screen, it does not prompt for
a new update. When I click on ‘check for updates’ IT STILL SHOWS 215.2470 (build 215.6354.561) as being
the latest version. The one that supposedly works is 215.2470 (build 215.6354.675)
Have you ‘Rebooted’?
Not getting the option to download Build 21.5.6354.675 via the Update section, either. Rebooted my PC several times and Build 21.5.6354.561 is still the latest build available.
Have you tried a manual program update (from the tray icon) ?
I just did that and got the latest build with the fix.
Tried via the System Tray icon, same thing. Build 21.5.6354.561 is still the latest version available. Even re-downloaded the offline installer, and did a re-install. Build 21.5.6354.561 is the version that’s installed.
Well the (manual program update) worked for me, however as I said I started of from avast free 2.4.xxxx.xxx, so I’m surprised it didn’t for you.
Others are also reporting they have got 21.5.2470 (build 21.5.6354.675).
Have you done a system restart yet (required for program updates), though I would have expected avast to ask for one.
I have build 21.5.6354.675 installed and no more popup error message “The server you are connected to is using a security certificate that cannot be verified. A certificate chain processed, but terminated in a root certificate which is not trusted by the trust provider.”
The Avast Add-in is listed as activated but Avast buttons are missing in my Outlook 2010, how to get Avast tools bar back ?
The same thing is happening to me…
I found a solution for me… just run online installer, reboot and build is 675
Some reports indicate a “Repair” will fix the issue (i.e. applying patch).
Open Avast → Menu → Settings → General → Troubleshooting → Repair App
After a repair, reboot is recommended.
The new Version 21.5.2470- Build 21.5.6354.675 isn’t shown after triggering a manual update.
You have to reboot your machine.
Emergency update kicked in today. Now on Build 21.5.6354.675, and can check my e-mail in Thunderbird without having to disable the Mail Shield.
Thanks guys 'n gals, it’s working again!
Thank you for the email fix!
Thanks, guys a lot for confirming the fix. Sorry for the complications with the update process. It gets delivered automatically, but to be applied to a running service the restart is needed.
In Windows 8 and later, Microsoft has introduced a hybrid shutdown, called: Fast startup. It works but logging out the user and then hibernate the machine. Next time all the kernel and services are restored from the saved memory image (the hibernate file) and the user logs in again - essentially restarting all the user-mode apps. This is why “Restart” and “Shutdown and power on again” are not the same. Similar issues can be seen with Windows Updates as well. See here: https://docs.microsoft.com/en-us/troubleshoot/windows-client/deployment/updates-not-install-with-fast-startup
Thanks again,
Lukas.
I just updated to 21.9.2494 and this is broken again. Anyone else seeing this issue ? Is there a fix on the way ?
This has started AGAIN! I am on Avast 21.9.2494 (build 21.9.6698.703).
I’m using the software Postbox v7.0.52 for accessing Outlook.com
Screenshot: https://ibb.co/nRW2hMH