Thank you everybody for reporting!
NethServer uses a squidclamav version built with c-icap support.
Also the web antivirus module has been tested only with version 7 of squidclamav.
So we decided to rebuild our package which now will override any squidclamav package from EPEL.
The NS issue is here: Can't install squidclamav · Issue #6774 · NethServer/dev · GitHub
The fix has been already releases, it should be available on the mirrors in the next 6 hours.