Homepod 1st gen doesn't reboot

Apple Home(kit) suggested a software upgrade for my 2 Homepods (1st gen). I followed the upgrade procedure and one works fine, the other one seemed to be stuck. On the display the + and - symbols flash white (simultaneously).

The hard factory reset (touching top screen while powering or after 5 seconds doesn't work. Although the screen had the swirling red circle, the two reset beeps are not generated and when releasing the touchscreen (even after more that 60 sec) switch back to the flashing + and - symbols.

I cannot find (Apple) support for the Homepod 1st gen as I live in the Netherlands where Apple never released the Homepod 1. In neighbouring Germany the device was sold by Apple, however getting (repair) support seems impossible. Something I've not yet experienced by Apple.