Hi Mihnea, thanks for reporting this. I've got a few questions, if you could answer them all this would be really helpful. A couple of people have said this lately and I'd love to find out why it happens.
1) Are you using the latest version of the app?
2) Please check inside Facebook if you have any permissions set for iGP Manager and let me know what they are, you can do this here:
https://www.facebook.com/settings?tab=applications
3) I notice your current account was created using Facebook, so is this the account you're trying to login with?
4) Is the Facebook app installed on your phone and are you logged in within the Facebook app on the same account?
5) What happens when you press sign in with Facebook on the phone?