UART2 was configured with the wrong SERCOM for the used pins (PB22 and
PB23). However, after changing the SERCOM from 3 to 5 that led to a
conflict with UART1 (used for the on-board WiFi). But the used pins are
also usable from SERCOM 3, so in the end I switched SERCOM5 and SERCOM3
around.
With this change, I was able to get examples/echo working.