Upgrading from Leap 42.3 to Leap 15.0... How hard can it be? Well, there was a bit of fighting necessary. One fight due to an encrypted root partition, another one due to NVIDIA and libGL.
I used the zypper upgrade method, from a text virtual terminal (after stopping X), which always seems safer to me than booting on a USB disk and hoping the partition setup is correct.
After a successful zypper dup, the first reboot led to an error like
Booting in rescue mode on a USB key and doing `cryptsetup luksOpen /dev/sda6 cr` worked, so no issue with crypto libraries or something, it was just systemd being stupid. I mounted the root partition into /mnt and edited its /etc/crypttab. Turns out the last argument on each line was timeout=0 (due to something I tried a long ago, it's supposed to work, but it doesn't seem to), which I replaced with none, and while at it I also replaced the /dev/disk/by-id device name with a UUID (found with `blkid /dev/sda6`), as suggested on a forum. After the usual `mkinitrd` and `update-bootloader --refresh`, and a reboot, it worked, it's now asking my for my crypto password! Much better. I'm not happy about systemd-cryptsetup's error handling here....
Next problem: I got a mouse cursor, but sddm doesn't show up, can't log in.
Clearly a GL problem. glxinfo says:
libGL error: No matching fbConfigs or visuals found
Using `strace -e file glxinfo |& grep -i libgl`, I found the problem.
Maybe this helps someone out there....