Comment 3 for bug 1816249

For me I can give some context:
- DELL XPS 9370
- Installed 18.04, dual boot, part of which I think meant that I needed secure boot (may be wrong)
- Attempted to upgrade Kernel to fix some bluetooth issues (known bugs) using UKUU
- Kernel install went OK, grub updated, but then on reboot the system complains that the kernel is not signed
- Attempted upgrade to 18.10, failed on same issue to new kernel not being signed
- But rebooted and checked kernel and it had updated OK to 4.18...
Thanks all I know... hope it helps.