Apparently I was too optimistic when I wrote that everything worked perfectly now. In fact Minetest is a very demanding game compared to the other eight games available on this server. It consumes the same amount of RAM, is much more CPU intensive than all other games together and it strains the underlying hard disk considerably. Most of the time this was no problem even with only 512 MB RAM, but in peak times the server was unable to keep up with the load. Since I wanted to upgrade again in a few months, why not now? There are still some games on my todo list that I would like to host for this project.
The facts: Now this XEN virtualized server provides 1 GB RAM, faster disks (RAID 10) and more CPU power (2 cores, Intel XEON). I will investigate in the near future what additional mods for Minetest can benefit from all this processing power without overworking the server again.