uefi – Secure booting does not turn off – HP ProBook G5

I'm trying to disable secure startup to be able to double-boot.
Whenever I turn it off and restart, Secure Boot activates again and does not allow booting.

Enter UEFI, disable secure boot and enable legacy mode …
I did it more than once I could count, and every time I go back to UEFI to check the startup options, the safe boot is re-enabled and the legacy mode is turned off again.

I also save the changes each time. No chance. Ideas?