Xbox help please? "Can't connect to Xbox Live Party."?

I've seen this one asked before in the archives, but I've encountered a different situation: both my friend and I have our NAT type Open, and we still can't connect to each other in a party. We used to, back when 1 vs 100 (season two) was happening, but more recently we suddenly can't, and no idea what has changed. Likewise, we're unable to join games with each other. Anyone have some suggestions what might fix this, other than the usual NAT problem?