Can't intall Ubuntu 23.10 on MBR disk using manual partitioning

[BertN45]

All mentioned disk are still Master Boot Record.
I like to install Ubuntu 23.10 (default install) on partition sdb5, but I can't select any boot disk. It will not continue without it and it only recognizes the installation flash drive with the /boot/efi directory as boot device :(.

I have 3 disks:

Partitions sdb
sdb1 extended partition (38GB)
sdb5 ext4 partition to be used for Ubuntu 23.10 (19GB), now used by Ubuntu 22.04 LTS
sdb6 swap partition (19GB)
sdb2 zfs partition (1TB)
sdb3 zfs partition (962GB)

sda is a sata-SSD (128GB) for caching the zfs partitions of sdb

nvme0n1 contains:
p1 swap partition (2GB)
p2 ext4 Partition used by Ubuntu 22.04 LTS minimal install (17GB)
p3 zfs partition (493GB)

I also tried it in a Virtualbox VM with one MBR disk and one ext4 partition (sda1) and there I could install Ubuntu 23.10 over of Ubuntu 22.04 LTS.
Maybe sda1 is OK, but sda5 is assumed to be GPT :) ??

ProblemType: Bug
DistroRelease: Ubuntu 22.04
Package: ubiquity (not installed)
ProcVersionSignature: Ubuntu 6.2.0-34.34~22.04.1-generic 6.2.16
Uname: Linux 6.2.0-34-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.11-0ubuntu82.5
Architecture: amd64
CasperMD5CheckResult: pass
CurrentDesktop: ubuntu:GNOME
Date: Mon Oct 16 22:18:31 2023
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/ubuntu.seed maybe-ubiquity quiet splash ---
InstallationDate: Installed on 2022-01-09 (645 days ago)
InstallationMedia: Ubuntu 21.10 "Impish Indri" - Release amd64 (20211012)
SourcePackage: ubiquity
Symptom: installer
UpgradeStatus: No upgrade log present (probably fresh install)

[BertN45 (lammert-nijhof)]

I had the impression that the extended partition caused the problem, so I decided to delete it and replace it by 2 primary partitions; a root and a swap partition. In the manual partitioning, I deleted the extended partition with its sub partitions. I recreated the root "/" partition, but automatically a second partition has been created with "/boot/efi". I was not happy, because I would loose my swap partition, so I pushed revert and that resulted in the partition table from the photo.

Since when does an existing MBR HDD need a /boot/efi partition?