ExpressVPN use a different set of credentials to log in to their website and to use their VPN with OpenVPN. This is why the credentials you use to log in to the ExpressVPN website will not work on your InvizBox 2.
Click the following link to retrieve your OpenVPN credentials:
Click on "Manual Configuration":
- Your OpenVPN credentials are viewable on the right, in the 'OpenVPN' tab. Use these for L2TP on the InvizBox 2 as well.
You can find out where to enter these VPN credentials on your InvizBox 2 in this article.