@edgararuiz thank you for your reply.
The status shows something contradictory to sudo rstudio-server restart.
So to answer your question, it gives the following:
● rstudio-server.service - RStudio Server
Loaded: loaded (/etc/systemd/system/rstudio-server.service; enabled; vendor preset: enabled)
Active: active (running) since Tue 2018-07-31 15:48:56 CEST; 2min 49s ago
Process: 5914 ExecStop=/usr/bin/killall -TERM rserver (code=exited, status=0/SUCCESS)
Process: 5916 ExecStart=/usr/lib/rstudio-server/bin/rserver (code=exited, status=0/SUCCESS)
Main PID: 5917 (rserver)
Tasks: 3 (limit: 4915)
CGroup: /system.slice/rstudio-server.service
└─5917 /usr/lib/rstudio-server/bin/rserver
Jul 31 15:48:56 filipe-desktop systemd[1]: Starting RStudio Server...
Jul 31 15:48:56 filipe-desktop systemd[1]: Started RStudio Server.
Which clearly isn't true as the restart or stop command shows no process found. So I assume that this is some sort of a bug or so with the new Ubuntu version.