Why does this permanent power solution not work?

As I have never gotten an email with a low battery warning (although I activated that option; in the Logs I can see the “low battery error”, but still no email) I’m working on a permanent power solution.

I tried using battery adapters like those:

But it doesn’t work. The adapter provides 2000mA, I tried both 5V and 6V settings, I checked the output voltages and the polarity with my multimeter - as soon as I put the battery pack in, nothing happens.

Any ideas what’s the problem here? (Apart from me being fed up with Nuki’s unreliability)

Thank you!