I have seen similar issues with Avast with the past few versions of the modpack, and have had to temporarily suspect Avast in order to install.
Today was a first though - Avast is now blocking the connection to all of the direct download sites! Examples screen caps below.
I'm familiar with the potential for false positives - but Avast seems to have encountered enough (presumably) false positives they have blacklisted the download servers as well.
Yes, I could of course temporarily disable Avast again, download, install, re-enable Avast - but that is not exactly good practice from a security standpoint, right?
I'll look into report this to Avast as well.
CudaOne
FYI - URL to report a suspected false positive (file or URL) for Avast: https://www.avast.com/en-us/false-positive-file-form.php