I'm experiencing same issue today. Was all working last night. I am using EISY. All of my devices stopped working with Alexa today. I am able to log into ISY portal. (I did have to reset password but all good now) I can still control from the administrative console and control devices, and my portal license is still showing as ACTIVE. Alexa can successfully discover devices but cannot control them. I tried removing some and it re-discovered them without issue. When I ask to turn on a light, after a short delay Alexa responds with "<Name of device> is not responding" I've rebooted everything. I am not using Alexa+. Alexa is still using the ISY Optimized for Smart Home V3 skill.