zdnet[.]com/article/secure-boot-certificate-updates-expiring-2026/
There are multiple certificates supplied by Microsoft, the hardware manufacturer etc., that work together, and the 1st of those to expire expired yesterday. Hopefully no one was impacted & any bootable USB stick you have/use still work. [This PC is now booting using the newer certificates so I tried an older Macrium Reflect USB stick & it works -- because of that I'm fairly confident all the others will too.]