Thanks for your answer!
I use it, because I am used to the Eclipse IDE and it is has a good file overview on the left side. Luckily I've found the error. I've edited sloeber.ino.cpp to:
#ifdef __IN_ECLIPSE__
//This is a automatic generated file
//Please do not modify this file
//If you touch this file your change will be overwritten during the next build
//This file has been generated on 2018-12-25 19:13:08
void callback() ;
#include "pwm_timer1.ino"
#endif
Now I have no error anymore. But I do not get any Serial output. It seems the Timer1 would never go into overflow. But I set it to 1/2 second so it should really call the ISR callback(), which should print me the millis() in an unsigned long.