I'm just reporting this, in case someone had a similar experience.

Before experimenting with Antix15 I put aside the SSD with Antix13 on it, just in case. I made myself a custum live USB and tried to install it on my test machine (10y-old acer laptop) with a spare normal hard drive. Everything worked fine, and since I'm very satisfied with Antix15, I decided to install it on the SSD. Before installing, I repartitioned the SSD with Gparted. One system partition, one swap-partition, nothing else on the drive left. Then I installed from the live USB, running cli-installer in a terminal, always choosing the default options.

I never experienced any problem with that particular hardware, but this time the freshly installed system wouldn't boot. I saw an error message To fix the problem I rebooted from the live USB. A simple didn't help. I had another error message, complaining about an unknown filesystem on the hard disk (I tried installing twice, first using ext4, then ext2, same error). Googling around led to the following solution. First I mounted the harddisk (from SpaceFM, in a root window). The mount point was /media/antix. Then in a root terminal
No error messages this time, and the system booted up happily. No idea what went wrong really.