How to force firmware downgrade

Well, you asked how to "force" it, so hopefully I'm not overstepping my bounds in sharing this with you. Usually I defer to the K-Team for a request such as this, DrB. You could first try re-downloading the OS and using that instead of an older version, just in case the copy became corrupted whilst downloading the first time. If it doesn't work after the forced installation, try the next-newest release version.

How to do it:
Place the kaos.bin file onto the USB stick, but at the root level - not inside any folder.
Insert the stick and switch the unit on into Browse Mode whilst holding down both Page buttons.
The display will read, "Booting and Burning from USB device".

Just put the KAOS you want to downgrade to on a Kemper formatted USB stick in the root directory and hold '<' and '>' keys.

Edit : Monkey beat me to it!

Quote from DrB

Latest firmware has crashed my Kemper.

Please open a support ticket. We will provide help to recover the Profiler, also, we would be interested in a backup of your Profiler, so we might be able to reproduce the crash and fix it.

Are you using a MIDI controller like the GT Mastermind or Liquidfoot?

Thanks for the ticket - I think I know what is wrong..
Please check version 4.2.2.12590.