I had a problem connecting my iPad Air to my AT&T Unite wireless hotspot. Even though I was 100% positive that I entered the correct wifi password, I repeatedly go an error on the iPad Air saying that the wifi password entered was incorrect. I was able to connect before my update to IOS 9.0.1.
Other devices were able to connect to the hotspot and I was able to connect the iPad Air to other wireless networks.
A friend who is an IT genius told me that many wireless networks have a BLOCK feature. It allows you to view connected devices and block any devices you want to exclude from your network. If your device identifies with the wireless network as blocked, you will get an Incoorect Password error message, even though you are indeed entering the password correctly. (I guess they think it is better to say the password is wrong than that YOU HAVE BEEN BLOCKED.
Although I did not place a block for my iPad Air, it identified as blocked after the IOS 9.0.1 update and no matter how many times I entered the correct password, I got the "incorrect password" message.
I turned the block feature off from the AT&T Unite Hotspot and was able to enter the password successfully and connect.
Hope this helps...