Cannot get my Topping D10 + Objective 2 amp working, need help

I cannot get my headphones to produce any sound and I do not know what to do.

I installed the required driver for the Topping D10, I connected the O2 and the D10 using RCA cables and I plugged in the USB of the D10. I power the O2 using a European adapter with the same specs as delivered with the device (red light burning when plugged in, also tried with other adapter). The volume in the toolbar has recognized the Topping D10, and the D10 also changes the displayed sample rate depending on the song played.

What do I have to do to get my headphones working?
You need another device to diagnose. Can you connect the D10 to another stereo or powered speakers?

Same with the headphone amp. Can you take the input from another analog source, like your phone?

The D10 does indeed work with another amplifier, while the O2 amplifier does not work when I use it with my phone and an aux cable as input (this should work if the amplifier worked accordingly right?). So my guess is that either the adapter I'm using does not work with the amplifier or the amplifier itself was delivered broken.

Thank you for the help.
See if the wall wart / power adapter is working. If you don't have a voltage meter try a replacement, even if the voltage or current is too low.
You could also try running the O2 with 9 volt batteries, without the wall wart. If the O2 works well running on batteries, then the problem is with the wall wart.