Decided to try once more. After the graphic problem was resolved by what pourpier wrote above I started the installation. Everything went well. I choose not to upgrade during the install, and said yes to all the default choices from the installer. No crash message. On the last installation screen when I choose to reboot everything froze for me too. Powered it off.
When I rebooted, it booted into the live iso again. Rebooted, and now after a rather slow boot, even for Virtualbox, it worked. Started a terminal and upgraded. Rebooted again and this time it were at a normal Vbox speed. Everything is looking normal and things seem to work as they are supposed to do.
Just so that there are no misunderstanding. This was the 32 bit LXDE R10 of wattOS.