Wi-Fi authentication problem after a night sleep

Hi! After installing Android 5.0.2 to my Sony Xperia Tablet Z, I am facing a strange issue with Wi-Fi connectivity. When at home my device is continuosly connected to wi-fi. But after several hours, basically I notice it in the morning, there is an "Authentication Problem" with wi-fi, and I need to reconnect it. In wi-fi settings I have set "Keep wi-fi on during sleep" to "Always", it doesn't help though. Any ideas how to fix it? Prevously, on Android 4 wi-fi worked flawlessly without any problems. Thanks.