I am currently running 8.1.2 on an iphone 6 plus. I have recently noticed that the internet became practically unusuable so i decided to reset my network settings. I tried signing back in to the same wifi I used on my other devices and it gave me the "incorrect password" error. So at first i thought of trying to forget and reconnect to the network on my iphone 5s(also running 8.1.2) to make sure i wasn't typing the weong password, but it worked. Next i tried to go into the router settings and copy the password onto my 6 plus but still could not connect. I've never gotten this problem before.
EDIT: I will try and update to ios 9, if it still does not work. I'm going to apple to exchange and get back on how it goes
Update: I decided to hard reset and restore my phone and the wifi started working again. The problem has been solved, i do not know how but i would appreciate anyone's answer might there be any future problems