@mtomek Hello,
I'm really sorry—I somehow overlooked the fact that you were posting in this old thread. The "Warning 1 1" indicates that the buffer for serial communication between the Master and Slave devices is full:
#define WARNING_SERIAL 1
#define WARNING_SERIAL_BUFFER_FULL 1
Have you added any new data communication or changed the communication speed recently? I’d also like to point out that REXduino is a third-party project, and we don’t have direct control over it. I found the explanation for the warning directly in the source code here:
https://github.com/jaroslavs/REXduino/blob/master/REXduino_slave/REXduino_slave.ino
Cheers,
Jan