Hello,

It seems that 15.3 was more stable than 15.4.
15.3 works, but not 15.4

Here the post with the error message during installation:

https://postimg.cc/gallery/WZ35np6

Kind regards

It seems you may have attempted to install in UEFI mode on a system with a disk partitioned for legacy booting. Please provide output from fdisk -l or parted -l. This can be done with a live media boot of any version of Linux. If necessary, use the Leap installation media and when you reach the license screen, do Ctrl-Alt-F2 to reach a standard command prompt. Command output can be redirected to a file on another USB stick, from which you could copy & paste here. This is preferable to a .jpg of a screenshot to postimg.cc.

my disk is a MBR with sda1 and sda2.
I just installed without efi, but visibly leap 15.4 crashes if efi is not activated.

another bug ...

What do you mean by "activated"? The choice to boot the installation media in legacy or UEFI mode establishes which mode the installation will be made in. There is no "activation" to be made. The initial boot menu differs according to whether legacy or UEFI was the startup method. In legacy mode, there is a row of Fn selections available for various boot options. It's not present if UEFI started.

In 15.4 you have the choice to use (U)EFI or not, still today. Likely it will change soon or later, to have only EFI, who really knows.
Boot Legacy, MBR was possible in 15.3.

During installation, I selected grub2 without EFI. (in 15.4 and as well, in 15.3 before trying 15.4)

The installer will pre-select the appropriate Grub for installation based upon the mode booted, and set it up. If you booted in UEFI mode, then selected to use non-efi Grub, I can only guess that the installer would switch its setup method to non-uefi mode. I've never tried such a broken path. The two Grubs' setup methods differ significantly.

Like 15.3, 15.4 supports both booting methods, but you make that choice at boot time by selecting which method to boot the installation media. After that, the default package selections are based on the boot method. You can choose quite specifically which software to install or not, but the default Grub setup path got determined before the installer started. If you tried to change it by any method other than restarting installation in the other mode, I can only imagine the likelihood of a successful bootloader setup by the installer to be between low and zero.

it seems that 15.4 just did work, while in 15.3 was super fine and working.

What does this mean? How did they differ?

I did same installation process, but 15.4. did fail with grub2 non-efi (i.e. legacy, mbr).
(see screenshot).

You may try installing 15.3, then live upgrading to 15.4 using the zypper package manager.