When trying to update the firmware of my M300 to any firmware mentioned at (here for instance the 450B13)

there will only be a 3 second redirect to the same page without any further action. No update is loaded or installed. The actual firmware is IPDECT/04.10/B0004/22-Mar-2018 14:28

I tried everything with reboots, setting back to factory settings etc. but it won't change a thing.

Does anybody have a clue for me what I'm doing wrong? Or is my M300 broken?

Kind regards! Felix