What does the form field »Enter Auth username« during the boot proccess mean?

I encountered this same issue and managed to resolve it. I first opened the OpenVPN init script with:

sudo nano /etc/default/openvpn

And uncommented the line:

AUTOSTART="none"

This should stop all OpenVPN connections on startup and thus resolves the issue without actually removing OpenVPN.

The problem was caused by some configurations file in /etc/openvpn. I don't know why the problem came only up after an upgrade to Ubuntu 15.04 as those files had been a long time before there. But actually, if you don't run an openvpn server or have some important openvpn configurations you can easily delete all those files with:

sudo rm -R /etc/openvpn/*

If you want to do a backup first, consider running something like this:

sudo tar -czf /etc/openvpn/