Bangle.js 2 stuck in bootloop. dfu flashing did not help

The instructions say

You should now have the Bangle.js logo, version, and MAC address on screen, and you can release the button

but my watch does not get into this state. It keeps rebooting with "Checking storage...
Storage OK" and "Watchdog.
DFU 2v12
Check storage.
No new FW.
Booting...".

My button is not stuck as I can switch to DFU mode by releasing the button while === are on the screen.

I already let the battery run flat and charged it but it is still stuck.