What I observed is that with devices that require a pin confirmation on the host, If I pair from the application (Bluetooth chat), the pairing completes apparently successfully, but without asking for PIN confirmation on the host. After that, on both client and server devices appear as paired, but connection is not possible. If I pair through the Android Bluetooth settings, everything is working as expected. Somehow the pairing mechanism used when trying to connect to a not paired device, does not work as expected.