Is Facebook login working for you personally? If so, have you reached back out to Facebook to describe the error they are experiencing? A few have come into support that were caused by third party items installed, manual IP bans, etc... My personal site got sent this because the Facebook tester was in a country I block on my personal server. Ultimately, getting the exact error from Facebook will help you narrow down what they're experiencing then if it is an application error, it can be possible to get the date/time or IP address that they accessed from to see what error they are encountering.