Thanks @teastain for the hint with USB CDC on Boot!
I forgot to mention that I am on PlatformIO but the snippet shown below added to the platform.ini file helped me to solve the problem. Previously I didn't had to specify any build_flags to see the Serial output, so maybe this is a change introduced by PlatformIO.
monitor_speed = 115200 build_flags = -D ARDUINO_USB_MODE=1 -D ARDUINO_USB_CDC_ON_BOOT=1Best
Marc