We have asked Avast what the problem is, but no reply yet.
In the mean time, to continue playing MTA you may have to add our URLs into the Avast exclusion list:
1. Open Avast
2. Click on 'Settings'
3. Click on Global Exclusions
4. Select 'Urls' tab and enter these, one at a time:
[url=http://*.mtasa.com*]http://*.mtasa.com*[/url]
[url=http://*.multitheftauto.com*]http://*.multitheftauto.com*[/url]