Comment 19 for bug 280160

I confirm this same bug in Ubuntu 12.04 LTS Precise Pangolin. The same workaround works:

Steps to workaround:

1. Change filename of /usr/lib/NetworkManager/nm-openvpn-service-openvpn-helper
cd /usr/lib/NetworkManager/
mv nm-openvpn-service-openvpn-helper nm-openvpn-service-openvpn-helper.dist

2. Create new file with same old name:
nano nm-openvpn-service-openvpn-helper

3. Fill the file with variables to export and original executable. Copy and paste this:

#!/bin/bash

[ -z "$ifconfig_local" ] && export ifconfig_local=$4
[ -z "$ifconfig_remote" ] && export ifconfig_remote=$5

/usr/lib/NetworkManager/nm-openvpn-service-openvpn-helper.dist $*

4. Make the new file executable:
chmod +x nm-openvpn-service-openvpn-helper

Now it works!