Hi Michel
This is curious. Although it's hard to tell whether AVAST is flagging the desktop or the site, it's message implies something wrong with the site. That being said, one option you have is to engage the Sucuri Labs group - labs@sucuri.net. They are always looking for interesting cases, and they'll be able to run a number of tests, above and beyond the SiteCheck scanner, to see if something is showing that might be triggering Avast.
The thing with AVAST is that it's concerned with end-point security, meaning it's looking for things that are triggering on the local environment (which could or not be coming from the specific site. )
Hope this helps.