galaxy s6 keeps restarting after installing stock firmware

[suhail807]

I upgraded my glaxy s6(G920S) to nougat and didnt like it so I decided to go back to marshmallow. I downloaded the marshmallow firmware from Sammobile and got Odin. I flashed the firmware and it said passed in Odin. But after that the device keeps restarting and gives messages like "contact storage stopped working"or context services stopped working. After this message the device restarts and this process repeats. pliz help