Simply updating to 5.6.2 solves the problem.
This question already has an answer here:
I recently stumbled upon problem when trying to login via facebook app login. First of all, I've already read every possible thread about problems with fb login, followed every possible tutorial but with no effect. Everything worked perfect when it was first done but I've just found out it's not working. I get this:
I get the message - domain is not included in the app domain field. But in fact it is as you can see below:
Help me Stack Overflow, you are my only hope.