The Qt doc tells the timer ID is -1 while timer is not running. So the ID is -1 in my constructor.
After calling start() the ID is a positive number.

When I create and set the interval of the timer in the run() method it works as expected. My GUI is constantly increasing the value.

So there must be a difference in timer objects created in thread constructors and timers created later (e.g. after starting the thread).