That's probably because you should always run your production servers on Long Term Support (LTS) releases of ubuntu linux
1. I think you should make a backup first and try the Ubuntu 12 package ,i would say that one have lesser chance to break something else on your system.
2. Or you can upgrade your ubuntu instalation to 14.04 and install the version for that one.
for the first option , try
wget http://swupdate.openvpn.org/as/openvpn-as-2.0.21-Ubuntu12.amd_64.deb
gdebi openvpn-as-2.0.21-Ubuntu12.amd_64.deb
# Note , if you don't have gdebi installed ... then install it "apt-get install gdebi"
# gdebi offers a better option to dpkg to install debian packages from the disk , because it also resolves dependencies.