Why is wifi on Android Nougat not working?

[AC Question]

I have updated to Android Nougat, but I can not make a connection to wifi-networks (to which I was previously connected) that require a login and password. When I enter my credentials (and all other settings) correctly, the 'connect'-button won't work (it stays grayed-out).
The only wifi-networks that work on my phone now are networks that are freely accessible for everyone and do not require an inlog and password.

How is this possible? Is there a setting that I have to adjust?

 

[B. Diddy]

Welcome to Android Central! Which device do you have? Have you already tried forgetting the network and reconnecting? Are you certain the password is long enough, and that you're not missing any characters?

I wonder if there could also be an issue with screen overlay. Do you have any apps that might use screen overlay, like a screen dimming app?