There is a bug in Arduino SAMD core that make prettyPrintTo() fail (issue #327)

It’s been fixed on May 27th by arduino/ArduinoCore-samd@8ef9bb3 and arduino/ArduinoCore-samd@8d0c167. Unfortunately, version 1.6.6 only includes modifications up to May 19th, so the fix is not available through the Arduino Boards Manager.

I recommend to apply the modification manually or wait until the next release of the Arduino SAMD Core. The files to change are USBCore.cpp and CDC.cpp.

They are located here on Windows: