I have been dealing with this as well for the past few weeks. I even tried using a static IP address but the authentication error would still occur. I stumbled across this solution and applied it early this afternoon and it has been 6 hours without the problem. Go into settings - WiFi and choose advanced. Find the option to have WiFi on during sleep. Change this to never. (Mine was set to always) Now restart your phone. Go back into settings - WiFi and choose advanced. Change WiFi to be on always during sleep. That was it. Let me know if it also solves your problem.
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