Thanks for your replies.
root@www:/var/discourse/shared/standalone/log/rails# free -h
total used free shared buffers cached
Mem: 994M 929M 64M 7.7M 4.6M 36M
-/+ buffers/cache: 888M 105M
Swap: 255M 191M 64M
I do not have any plugins installed. It’s still the very default Discourse from your installation guide. The only thing that I can mention is that I at first tried the 512MB server from DigitalOcean and increased to 1GB when installation failed and then started once more.