Hello Jediy
Had the same problems with various yamaha musicast devices. After the firmware upgrade I could only pair a few speakers. Tried x things and finally I plugged all devices that were further away from the WiFi access point into a network cable. After that the pairing worked fine again.
According to a forum post, yamaha has changed something in the transmission encryption and if a device is now too far away from the WiFi AP, the delay for synchronisation is too long. ...for me, the sync only works perfectly again if the distance is less than 5m...tragic, but that's the way it is.

I had temporarily switched to sonos out of annoyance with yamaha...but the app is even worse there than with yamaha...in my opinion.

If both could programme a decent app, both systems would be top.