Hello everyone, I am writing this short post to help anyone who is facing this warning message during the system boot: I finally managed to permanently solve this - even if it’s mostly harmless.
I have a 10yo ASUS X555LJ, always used in dual boot mode. In order to keep it alive because of the October departure of Windows 10, I have decided to do a clean install of MATE 24.04. After everything was set up, I kept getting that annoying message.
After following thousands of guides on the internet (including from other MATE-based distro communities), all of them claiming that the issue would go away after re-enrolling security keys and disabling Secure Boot in UEFI security options, but I kept running into the same problem.
In fact, after the first peaceful reboot, the warning came back - again and again.
But probably it was because of my computer UEFI does not allow me to disable Secure Boot anymore (considering I did disable it once back when I first installed Ubuntu in dual boot with Windows, so 
).
Finally today, I noticed a disabled option: Launch CSM. I decided to give it a try, enabled it, did the key re-enroll process again... and boom! The message hasn't appeared since.
I hope this helps and is not just a useless post. Bye! 
