Could not reprogram device because of connection problem

[Appartus]

Hi 
I'm currently develop new product based on RAK8212. I have problem with reprogramming devices. Programmer i use is nRF52-DK
I'm not able to reprogram about 10% of devices. Its not happen at first try. Boards stop responding after few programming cycles but connected to power supply works with previously uploaded firmware.

Log in attachment

log.log

[haakonsh]

What's the target voltage of the nRF devices that you're programming?

[Appartus]

I'm using 5V DC power supply and Li-Po; 3,7V; 300mAh battery

[haakonsh]

Yes, but what is the actual voltage supplied to the nRF device when you're programming? 
I ask because the on-board debugger only supports ~2.8V. 

[Appartus]

Programmer is powered up from USB but I don't think that is problem with programmer because if I will connect different device there is no problem with flashing. 

[haakonsh]

But what's the target nRF device's voltage, the device that you are trying to program? 

[Appartus]

VDD nRF -> 3,3V

[haakonsh]

You will probably have to power the VDD net of your custom device with 2.8V while programming it, as the OB J-link only supports 2.8V