Are there any possible issues or side effects if we disable IPv6 on dsfw servers ?
Let's say: at least i don't expect any. Merely the first thing i do on any OS is to disable or at least unbind IPv6, so DSfW will happily work without any traces of it. While this applies to offsets which have never seen IPv6, i can't think of anything weird which could happen if it had been there once and torn away later on.
Are there any trust relationships with Windows domains which are v6 only?