I'm having an issue on my Nexus 4 (Stock Android 5.1.1).
It keeps disconnecting from my Wifi. When I go back to connect, it says "No Internet access, automatic connection impossible".
I currently live in China so my guess is that Android checks if internet is available by querying a Google server, which is blocked in China so it then concludes "No Google = No Internet" and disconnects me from my Wifi.
I can't find any way to disable that Auto-Disconnect "Feature".