NodeMCU esp12e V2 Vin trouble

Hey guys!
I have some trouble getting my NodeMCU esp12e V2 to work with a supply on Vin Pin. With USB everything works fine but as soon as i connect an external power supply to Vin (tried different voltages, amperes etc) nothing happens. I checked the wiring of the onboard VoltageRegulator. everything is ok.

Only possibility to get it to work with solely Vin as powersource:

-Connect power to Vin
-Connect USB
-press RESET
-disconnect USB

any ideas?
i think it has something to do with the diode connected between USB VDD and Vin.....