I followed this thought a bit more and looked at the initrd-files in /boot. I found that the one for the working older kernel is bigger than the ones for the newer not working kernels. I compared the content of the working kernels initrd file /boot/initrd.img-4.19.0-2-amd64 to a newly generated initrd-file for the same kernel and found that there is a lot of difference.
Some of it obviously was related to use and initialization of the gpu and acpi. Also files for plymouth were missing. I remembered that I purged plymouth, because I’m not using a graphical boot screen and have the (bad?) habit of deleting software that I think I do not use.
I installed plymouth and related packages and tried again and the difference related to gpu and acpi not existed anymore.
Yes, I can now start my Librem13v4 with the actual kernel (and the Debian kernel 4.19.0-6-amd64 worked also. Both with newly generated initrd-files.
I’d suspect that there is somewhere a dependency on plymouth is missing. If this is related to the use of PureBoot/coreboot the (meta) package containing the dependency for PureBoot is missing (could also help with documentation issues: containing files in /usr/share/doc/ and could provide the current coreboot_util.sh).
So which part of the boot process does depend on plymouth?