...I found the explanation: https://www.visualmicro.com/page/User-Guide.aspx?doc=Working-With-Breakpoints-Wh...

...now it works

Sorry, the serial debugger is not a full debug solution and relies on you stating the variables (or method return value) you want to watch (and/or update) before upload. It does not show data tips for variables it only shows data in the watch window and trace.

The serial debugger avoids the need to add serial.print debug to your code, does not store the messages using the micro controller flash and can plot charts, show min/max values, use timed messages and other things hardware debug can not do. So it can be a quick and easy solution otherwise you have to use the hardware debug option but that is more complicated for avr.