Yesterday I was reconfiguring my Wi-Fi network with settings like strenght of signal, channel which AP should transmit etc. After work has been done. All iPhones couldn't connect to long known network. Even after forgetting network in the device I was unable to connect again. Typing password always gave the message "incorrect password".
After some research (and it was not changing Wi-Fi password or SSID name :) I've found that Apple is handling beacon timing of power saving differently than other vendors.
Default value in Unifi Controler for DTIM Period is "1", and Apple recommends it to be "3".
To change that one have to login to controller, then go to Settings:
Then go to Wi-Fi, then Wi-Fi Networks, and on your network click Edit
Find 802.11 Rate and Beacon Controls and enable Override DTIM Period
Change default value of 1 to 3 and save. Wait until new settings are propagated to your APs and you should be able to connect normally.