I tried 3 different USB sticks and could not revert back from 7.5 beta to 7.41.
Got the same message.
Try the 7.3.2 bios. Its available for download in the 'older software versions' of the download page.
When my usb stick failed and borked my kemper during the recent 7.4 update debacles, I put this version on, and then let Rig Manager handle the updates.