Based on this lilo.conf, I'm suspecting you have a problem with adding the Slackware entry to it. You have Slackware's kernel listed as /boot/vmlinuz-huge-4.4.14, but this is typically where it's stored when you are booting Slackware. You would need to reference the kernel location from within your debian install. This would require you to mount the Slackware partition, then provide the full location to vmlinuz-huge-4.4.14.

If you have /boot on a separate partition, you'd need to mount that, but if it is on the root partition, you could mount it something like mount /dev/sdb2 /mnt and then edit your image location for the Slackware entry in your /etc/lilo.conf on your debian install to read image = /mnt/boot/vmlinuz-huge-4.4.14 and hopefully that will fix your lilo issues.

But as others have said, while lilo may be the default, it also supports (and includes) grub2 and the installation disk uses syslinux. Slackware is all about making the system your own, so feel free to get it working on grub if you'd prefer to use that, but we can certainly get things working with lilo if you'd like

Setting up GRUB 2 on install (without rebooting)
https://docs.slackware.com/howtos:sl..._first_install

Thanks for your answers!

Well ... I now use lilo since it was automatically (AFAIR) installed. Among the next things I'll do is to get X started. But that's another thread.

Regards,

Gregor