boot – Post kernel update: Only OVH’s rescue kernel works – but why doesn’t mine?

I just went through upgrading from Debian 9 to Debian 10 and I am having a weird issue. Probably not too weird to people that do more intensive sysadmin things, which is why I bring it here:

During the whole upgrade procedure, I also upgraded my kernel to this:

$ dpkg -l | grep linux-image
ii  linux-image-4.19.0-12-amd64       4.19.152-1                                       amd64        Linux 4.19 for 64-bit PCs (signed)
ii  linux-image-amd64                 4.19+105+deb10u7                                 amd64        Linux for 64-bit PCs (meta-package)

However, when rebooting, the server just doesn’t go online and instead triggers Kimsufi (OVH redistributor/reseller) automated warning that there is an issue with the server (as if I wouldn’t know that after waiting a hour for a reboot…). Their automated response reboots the server into a “rescue mode”, or more specifically, into this:

$ uname -r

Obviously, the kernel I have installed (4.19.0) is actually older than their rescue kernel (4.19.62). But I doubt this is the issue. …or is it?

How can I, while booted into the rescue kernel, figure out what prevented my maschine from booting? /var/log/boot.log doesn’t exist and messages only has log messages from the rescue kernel boots – none hinting at an attempt to boot mine.

For completion, here is a gist with the GRUB config that was generated:

Because harddrive schemas aren’t unimportant for booting:

Device     Boot      Start        End    Sectors  Size Id Type
/dev/sda1  *          4096    1050623    1046528  511M 83 Linux
/dev/sda2          1050624 3863281250 3862230627  1.8T 83 Linux
/dev/sda4       3905972224 3907018751    1046528  511M 82 Linux swap / Solaris