@ptillisch The problem was not solved with changing the standard settings nano_nora.serial.disableDTR and CTS from false in 2.0.17 to true.
But I remembered an old trick: create a new user account and install the program in there. And that worked! Serial communication was working again.
But in my own account the problem was still there.
Uninstalling and reinstalling the IDE1 and IDE2 did not work.
Also renaming the IDE folders in c:\Users\\Appdata folder did not work. The IDE still remembered the recent files et cetera.
But deleting the folder c:\Users\\.arduinoIDE\ solved my serial communication problem.
Maybe a nice addition in the IDE. ‘Reset all settings’