Hi Matteo,
Report to virus@avast.com and ask for an exclusion (refer to this thread here). They could consider that, I cannot as unblocking websites is only reserved for avast team members, I am just a volunteer here with relevant knowledge. Anyway you considerably improved your website security by reporting here. Stay secure with Avast!
Damian
P.S. Joomla scan OK: https://hackertarget.com/joomla-security-scan/
Note that this site: -http://www.open-society-kz.org/modules/mod_roknavmenu/themes/basic/code.php
had a threat identified as: Exploit.HTML.IFrame-6
pol