Bose Soundlink Mini 2 - Blinking red with varying PWM

[goaty]

I got a Soundlink Mini 2 which is blinking red in a strange way.
It starts with medium brightness, then slowly dims the blinking toward dark, then starts with full on and repeats the cycle.
I checked the battery and it´s fine even under load.
Behaviour is same when powered through USB or power supply connected to desoldered battery pins.
(USB connection to PC is not working, only Power coming through -> No Firmware update possible)

Anyone can give me a hint, what the Bose is telling me with the 'PWM' style blinking ?
I can measure the signal is coming from the STM micro.

Thanks, Thomas