Other issues I discovered:
* connecting the device after fully booted puts it into "permanent charging mode", read: the thunderbolt icon does not disappear. It says "charging (USB)" in the battery stats even when disconnected
* connecting the device while turned off, then booting, puts it into "permanent no-charging mode", read: the thunderbolt icon never appears, the battery does not charge
In both "modes" connecting to a wall charger changes nothing. A full reboot is needed. Then it works flawlessly with the wall charger unless I dare to use a PC for charging, that's when it returns into sticking with the thunderbolt icon. It will fully charge while in this state until I disconnect the device. Then I'm stuck with continuous discharging until rebooted. This is reproducable in recovery mode to where - according to dmesg - it acts the same.
But back to the core of the problem: I can no longer connect with ADB. This worked before. And I'm using Linux. So it is none of those strange Windows drivers problems.
Somehow I feel it might be a software issue (and I hope its not a hardware issue). The only problem to maybe resolve the issue is either reflashing factory in fastboot (which would work) or sending in for repair. Both options I do not like because I cannot backup my partitions.
BTW: The device is stock ROM, no root.
Anyone heard of such problems?