LL6.0 installs but after update get X.509 error @boot; LL6.6 install screws PC

[stevef]

Reformating an unused partition should be trivial.

It seems that at post #2 you had a 'working' dual boot system but with something unusual going on before the grub menu and in the logs.

I still get the X.509 error flashing up at boot before the grub menu. This is followed by "Error checking in progress". After about 10-15 seconds I get the Grub Menu

But, from the log posted in post #2

feb 05 18:31:48 Lenovo-LL kernel: integrity: Loading X.509 certificate: UEFI:db
feb 05 18:31:48 Lenovo-LL kernel: integrity: Problem loading X.509 certificate -65
feb 05 18:31:48 Lenovo-LL kernel: fbcon: Taking over console
feb 05 18:31:48 Lenovo-LL kernel: integrity: Error adding keys to platform keyring UEFI:db
feb 05 18:31:48 Lenovo-LL kernel: integrity: Loading X.509 certificate: UEFI:db

The inference from posts #1 and #3 is that converting the ext4 data partition (or maybe updating kernel) triggers the system to fail to boot with a specific x509 error.

Integrity Problem loading X.509 certificate -65

In your position, I'd be tempted to concentrate on getting the dual booting working properly by resolving or mitigating the x.509 -65 error before grub.  The suspicion is that it is related to UEFI and secure boot and possibly kernel update post install.

Edit - addition
Can't find anything particularly concrete, but there are suggestions that the -65 error might be related to disk problems.