Hot restart problem initializing Serial connection

[SimonSchw]

When I try to use the reset button to restart the board, or the board resets after uploading a sketch, the serial connection to the receiver fails to initialize, making the sketch get stuck in the setup method at IBus.begin(Seial1). Disconnecting the power from the board and trying again works most of the time, as does disconnecting then reconnecting the flysky receiver.

I use an Arduino Mega R3, receiver is hooked up to Serial1 for Servos and Serial2 for telemetry.