Virtualbox fails when I'm connected to an OpenVPN server

Some days ago, VirtualBox started to fail when I'm connected to a VPN. If my computer isn't connected to the VPN, everything works fine. I can start VMs, stop them, and so on. But if my computer is connected to the VPN, I get these messages:

$ VBoxManage controlvm myvm pause
VBoxManage: error: Failed to create the VirtualBox object!
VBoxManage: error: Code NS_ERROR_ABORT (0x80004004) - Operation aborted (extended info not available)
VBoxManage: error: Most likely, the VirtualBox COM server is not running or failed to start.

This is my configuration:

• Mac OS X El Capitan
• VirtualBox 5.0.26 r108824
• Tunnelblick 3.6.7c (build 4606) as VPN client
• OpenVPN server 2.1.3

Just in case, the OpenVPN server is not installed on my computer. It's on a server in my new office.

What would be the problem?

EDIT: I just connected to a different VPN and VirtualBox worked with no problem. Now I'm sure the problem is caused for some configuration in the OpenVPN server.

In my OpenVPNAS server, I deleted the value on "Configuration > VPN Settings > Default Domain Suffix (optional)" and VirtualBox worked again.: