well, once red all the posts in this tread, it seems that "incorrect password" is more a syntom that an illness. I mean, a lot of reasons may produce this unique error message.
I will explain my own problem and its solution: I have 2 iphone 6 plus and 4 ipad (1,2,4 and air 2). My router is dual band: 2.4 and 5G: different SSID´s but same password on both bands. All on ios 8.1, except ipad 1 of course,which is in ios 5 (thanks, Apple). Problem arises when i checked out that all devices (even ipad 1, LOL) were connected to 5G band, except ipad 2, which was connected to 2.4, and refuses connect to 5g: "incorrect password". Tried all possible solutions: reset router and ipad network settings, WAP2 mode, turn both off and on, chage password, change SSID, checked out MAC filtering, reset ipad general adjust settings, with no results at all. Even got no results when i turn off security of my wifi, with free access, which at last gave me the beginning of the possible solution: if it was not mater of passwords, should be the wifi itself. And then i found out the solution: the channel on the 5G band, (which was set on 100 mHz). I changed it to 40, not the lowest, but near., and problems went out. All devices connected.
So, my solution: change you wifi channels to low values in 5G band, and checked out interferences on 2.4 band.
one question remains: why 3 ipads and 2 iphones do connect to 100 channel on 5g and 1 ipad doesn´t