Thanks for your swift reply! I already suspected that it was not a good idea to use the normal port and the USB port at the same time. 
Just found an old 2A adapter to charge on the USB-C port. Seems to work smoothly so there's no shortcircuit of any kind I suppose.
Might replace the jackplug with a USB-C plug to provide 3A and stop wondering what is wrong with the normal charging port.
Just found an old 2A adapter to charge on the USB-C port. Seems to work smoothly so there's no shortcircuit of any kind I suppose.
Might replace the jackplug with a USB-C plug to provide 3A and stop wondering what is wrong with the normal charging port.