Now I updated the firmware (with corboot_util.sh I got pureboot-librem_13v4-Release-15.rom). I applied it successfully. But still the boot is interrupted: Now I have two more lines:
[ 0.935811] DMAR: [DMA Read] Request device [00:02.0] fault addr 85c4db000 [fault reason 06] PTE Read access is not set
[ 0.952456] DMAR: DRHD: handling fault status reg 3
[ 1.589397] i915 0000:00:02.0: firmware: failed to load i915/kbl_dmc_ver1_04.bin (-2)
[ 1.589400] firmware_class: See https://wiki.debian.org/Firmware for information about missing firmware
[ 3.043477] sd 3:0:0:0: [sdc] No Caching mode page found
[ 3.043503] sd 3:0:0:0: [sdc] Assuming drive cache: write through
BusyBox v1.30.1 (Debian 1:1.30.1-4) built-in shell (ash)
Enter 'help' for a list of built-in commands.
(initramfs) _
So just the two lines with [sdc] are new … but are they any useful information?