It turned out my Ubuntu server does not start Subversion automatically on boot. I found this website with instructions how to create a startup script.
Create a file called “svnserve” in the /etc/init.d folder
$ sudo nano /etc/init.d/svnserve
Add the following line to this script in order to start the Subversion server:
svnserve -d --root /srv/subversion
Save the file (CTRL+O) and exit nano (CTRL+X)
Now make the file executable
$ sudo chmod +x /etc/init.d/svnserve
Add the svnserve script to the init.d boot sequence
$ sudo update-rc.d svnserve defaults
Now you’re done! On the next reboot the SVN server will start automatically.