I have seen this happen when a IPO500 expansion loses connectivity to the primary server (where the server edition licenses reside)... what happens then is that the IPO500 ends up running minus even an essential license (depends on whether the IPO was originally built with an essential license or not... if it's a brand new addition, there's no reason to get an essential license if it's going into server edition).
Anyway, if this happens, twinning stops working.
Happened to me twice, when customers have made routing changes that stop one site or another from effectively reaching the primary server, and being able to pull licneses. traffic was one way only on those links... I don't remember which wway off the top of my head.
You can check this by logging into the various sites (primary and the IP500 in question, if that's where the issue is), and checking under status / SCN licenses and also Small Community Networking.
GB