Constant avast messages about mail shield security exclusion

Hello,

I am using a pc with Windows 7 ultimate.
The last few weeks an avast window keeps popping up saying “mail shield security exclusion” (see picture below) and says that there is a problem with a site certificate. This will go on every 5 to 10 minutes and never seems to stop.

They pop up even if all web browsers are off.

Any advice is more than welcome!
thanks,