SzymekRak:
But, unfortunately, the UART1 pins are also interrupt pins. But I think I won't need to use more than one in my project.
The interrupt problem is even worse: No matter which interrupt I use, attachInterrupt defaults always to INT0 on pin 16/D10 - which is now used for RX1 on UART1. So in fact one only of the three interrupt lines is usable.