Setting up VPN Server (PPTP) on Server 2012 R2 with Android phone VPN access

RRAS using PAP (Password Authentication Protocol)

In case your RRAS VPN server is set to use PAP protocol, then during configuration of Android VPN client at Settings > More connection settings > VPN > tap Add VPN, you have to deselect PPP encryption (MPPE), because PAP protocol does not support that encryption.