boot stuck on reboot due to microcode loading

This is happening on a Notebook I just set up: Medion Akoya E6424
CPU: Intel i5-6267U

The BIOS is from 2016, but no more recent version seems to be available.

What's interesting:
Cold boot usually works, with the microcode successfully loaded.
In this cases, I get e.g. this log ouput:

Nov 18 11:58:20 PC1 kernel: [ 0.000000] microcode: microcode updated early to revision 0xe2, date = 2020-07-14
...
Nov 18 11:58:20 PC1 kernel: [ 0.097689] SRBDS: Mitigation: Microcode
Nov 18 11:58:20 PC1 kernel: [ 0.097690] MDS: Mitigation: Clear CPU buffers
...
Nov 18 11:58:20 PC1 kernel: [ 0.571739] microcode: sig=0x406e3, pf=0x40, revision=0xe2
Nov 18 11:58:20 PC1 kernel: [ 0.572539] microcode: Microcode Update Driver: v2.2.

However if I reboot the machine, it gets stuck while booting. Nothing is written to the logs in this case, making me guess that it fails very early, maybe in the bootloader itself.
The only workaround I could figure out was to disable microcode updates. I figured this out because the rescue boot option works, and the relevant difference seems to be the kernel parameter "dis_ucode_ldr".
From there I figured out that I could disable microcode loading via the config option IUCODE_TOOL_INITRAMFS=no in /etc/default/intel-microcode. Adding dis_ucode_ldr to the kernel paramters for the default boot option would also have worked.

This are the relevant log entries when microcode updates are disabled:

Nov 18 12:59:54 PC1 kernel: [ 0.025754] [Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version: 0xb2 (or later)
...
Nov 18 12:59:54 PC1 kernel: [ 0.088777] SRBDS: Vulnerable: No microcode
Nov 18 12:59:54 PC1 kernel: [ 0.088778] MDS: Vulnerable: Clear CPU buffers attempted, no microcode
...
Nov 18 12:59:54 PC1 kernel: [ 0.546881] microcode: sig=0x406e3, pf=0x40, revision=0x6a
Nov 18 12:59:54 PC1 kernel: [ 0.547789] microcode: Microcode Update Driver: v2.2.

Since I now have a reliable way to boot the machine, it's not an urgent issue for me anymore. Also, I'll not be able to do much further testing, bcs I'm now handing over the machine to somebody else.
Anyway, I wanted it reported, maybe that can save others some troubles.

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: intel-microcode 3.20201110.0ubuntu0.20.04.2
ProcVersionSignature: Ubuntu 5.4.0-54.60-generic 5.4.65
Uname: Linux 5.4.0-54-generic x86_64
ApportVersion: 2.20.11-0ubuntu27.12
Architecture: amd64
CasperMD5CheckResult: skip
Date: Wed Nov 18 13:09:27 2020
InstallationDate: Installed on 2020-11-14 (3 days ago)
InstallationMedia: Ubuntu 20.04.1 LTS "Focal Fossa" - Release amd64 (20200731)
ProcEnviron:
 LANGUAGE=de_AT:de
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=de_AT.UTF-8
 SHELL=/bin/bash
SourcePackage: intel-microcode
UpgradeStatus: No upgrade log present (probably fresh install)
mtime.conffile..etc.default.intel-microcode: 2020-11-18T12:57:12.704592