I’ve run into this problem as well. I can’t update my kernel as the linux-image-6.12.5-mnt-reform-arm64 package in the mnt repo has been renamed to .deb.off so I get a 404 when trying to apt upgrade. I’m not familiar enough with apt to know what to do here.
I have linux 6.8.11 packages still installed from the last time I did an upgrade. Is there a way to get apt to swap back to that older kernel?