The problem has been solved. It was a configuration problem. And there'll probably be a patch for Marlin which prevents this problem from happening again in the future.
The Configuration.h I used included these 2 values:
#define SERIAL_PORT 3
#define SERIAL_PORT_2 1
Since I changed them into
#define SERIAL_PORT 1
#define SERIAL_PORT_2 3
everything works as it's supposed to.
The Configuration.h I used included these 2 values:
#define SERIAL_PORT 3
#define SERIAL_PORT_2 1
Since I changed them into
#define SERIAL_PORT 1
#define SERIAL_PORT_2 3
everything works as it's supposed to.