I can't see how the SatNav showing you are 1/2 a kilometre off where you are could be related to avast, even if it were blocking it when you trying to update your SatNav.
Having disabled the firewall without any difference, would tend to support it isn't the firewall, that leaves the Web Shield and or the File System Shield. The Web Shield being the more likely of the two, but if it was that then I would expect to see an alert or interactive window from avast.
Trying to pinpoint the file that is responsible for the update is what you need to find, then you could add that file name and its location to the File System Shield and add the URL (it connects) to the Web Shield exclusions.
In your firewall settings, which setting do you have it set to, if it is too rigorous it could block (but as you said disabling didn't work) ?