Disabling avast leaves you completely unprotected, this is why I suggested disabling the mail shield as a minimal step and to see if that worked.

That said, since you are only downloading the headers, I don’t know if the program you use for that (?) is actually using NNTP protocol.

I wonder if you can add the usenet reader (executable file and path) to the avast Exclusions. However, as you say you “told avast to ignore newsbin everywhere I know of” and those exclusions might exclude scanning the file, but not its actions.