Aircard 785 retail - boot loop and no update possible

Hello community, I got an aircard 785 on ebay - just thought this would be an easy problem to fix, but...
Whenever I turn it on, it shows me the starting screen and reboots. This continues in a loop for some minutes and finally the device enters the "Status LED blue/amber blinking" state. Connecting the AC to the PC and trying to update the firmware by using the software "AC785S_02.08.00.51.00_EMEA.exe" (linked as "older version" on the product support page) seems to work as intended: the screen shows "Downloading software update" and then the icon "Installing software update". After that, the device just enters the start screen and loops... and finally enters the blue/amber flashing loop again.

When I use the firmware updater "2 - AC785-100JPS_WU_update", the icons are barely readable and the "starting screen loop" starts immediately.

So this feels like an incompatible firmware to me. Is there any archive with other flashable firmware to try out?

Cheers,

T