Kobo Clara HD (N249) — USB not detected for data, charging works fine

[Clemg35]

Hi everyone,
I have a Kobo Clara HD (model N249) that charges normally via USB, but is never detected as a storage device by my PC (tested across multiple ports/cables/PCs, including a cable confirmed working on another device). Wi-Fi and the Kobo store work fine, so the device is otherwise fully operational.
I opened the e-reader and took some multimeter measurements (DC voltage, device powered on):
VBUS pin (right): 5V
2 middle data pins: 0.6V (both)
2 GND pins (left): 0V
Test points TP19/TP20 (right next to the connector): also 0.6V, consistent with the data pins
Based on the official iFixit USB Port Replacement guide, a loose/disconnected pin should normally read 0V — not 0.6V on both sides. So I'm wondering whether the issue is really the micro-USB connector itself, or something further downstream (ESD protection chip, USB controller) on the board.
My questions:
Does 0.6V on D+/D- at idle sound normal or abnormal for this kind of board?
Has anyone run into this exact symptom (charging OK, data never detected) on a Clara HD/N249?
Does a schematic or test point pinout exist for this board that I haven't been able to find?
Before attempting a blind resolder, would you recommend a continuity test (Ohms mode, device off) between the connector and a point further downstream instead?
Thanks in advance for any help!