Check the Group Policy Editor, Local Policy and in the case of a Domain Controller, Domain Controller Policy. See if anyone has mucked about with that.
AND
I am not a big fan of McAfee's anything and when I start having seemingly unexplainable network issues I always suspect the Security Suite. Generally I'll uninstall, get things up and running and then reinstall.
If all the "Servers" are running this then you might look into how they are configured as to what they do and do not trust.