Cannot update D3 firmware (UXM)

Hello Snom Community! I'm new here, nice to meet you.

I bought a D3 expansion module on eBay and connected it to my D385. My phone prompts me that the D3 needs a firmware update. But I cannot update it; the log says [ERROR ] PHN: UXM update failed because UXM has power supply connected but of course I followed the instructions: the D3 does not have a power supply plugged in and it is the only USB device connected to the phone.

Incidentally, the D3 itself will say Please connect to a compatible snom phone with firmware ≥ 8.9.3.28, but only if it powers on together with the phone. If I disconnect the USB cable and reconnect it, this message will not appear. Also, the D3 buttons do actually respond to a key press, but the screen and the LEDs stay as they are at startup: no LEDs light and the screen shows no pertinent information.

If I initiate the D3 firmware update from the phone itself, it will show the message that it is updating the firmware; 10 seconds later the [ERROR ] PHN: UXM update failed because UXM has power supply connected is printed to the log, and after a total of 30 seconds, the phone goes back to its idle screen without any further information. The same happens when I iniate the firmware update from the phone's web user interface (where I supply the URL to the D3 firmware; I only see the latest on the Snom site).

My current D3 firmware is v1.5.2.

I initially tried with the latest firmware on my phone (10.1.198.19) but I downgraded it to 10.1.33.33 which is the oldest firmware I could find (my hypothesis was that the D3 firmware was too old to properly support the power supply connected check of the phone's firmware). This made no difference.

A small excerpt from the phone's log when trying a firmware upgrade:

[DEBUG2] PHN: UXM update script version: 2, min required script version: 0
[DEBUG0] PHN: UXM update: Execute: /tmp/d7d3_update.sh '3xx Expansion V1.5.2' 0 2>&1
[DEBUG0] PHN: UXM update: Status: 0
[DEBUG2] PHN: UXM update: step 0 done, version info:3xx Expansion V1.5.2
[DEBUG2] PHN: UXM update: step: 1, version info:3xx Expansion V1.5.2
[ERROR ] PHN: UXM update failed because UXM has power supply connected

Another thing I tried is to set <telnet_enabled> in the phone's config, but neither the latest firmware nor 10.1.33.33 accepted this key, so I couldn't poke at it further.

Does anybody have an idea how to get the firmware updated? Note I'm fine with applying a soldering iron to the D3 if the components are not too small for my shaky hands.

Thank you for reading!

PS: I bought the phone on eBay as well; playing with its possibilities is just a hobby to me. I cannot ask my reseller because I didn't buy it at a Snom reseller .