I don't know why are you getting that error, but this is what I would do in your situation
sudo rstudio-server stop # Stop rstudio-server
sudo apt purge rstudio-server # Completely uninstall rstudio-server
# Download and install again
wget https://s3.amazonaws.com/rstudio-ide-build/server/trusty/amd64/rstudio-server-1.2.1572-amd64.deb
sudo gdebi rstudio-server-1.2.1572-amd64.deb