I do not know if it requires authentication (pleanty of altbinz mweb users, somebody will reply for sure).
My expirience with Avira is is that it does not scan nntp traffic (but some do and it's messing up altbinz)
Is group selection required for mweb? Have you tried without group select command? Also did you try with some other nzb?
What's your global connection timeout?