Really a malware? AVAST detects threads but scanners does not detects any virus.

Suddenly a subsite in my multisite network started to show AVAST alerts (see attached files)

The thread is detected only in one subsite (www.miraiserver.com) but no in the main site (www.bloserver.com) nor other subsites.

I scanned for viruses in cPanel, also checked the site using http://www.sucuri.net but no virus are found. Also scanned my mac for viruses using AVAST but no result.

Is anyone experiencing the same issue?

Thank you for your help.