Very recently out of no where I couldn’t log on to my WordPress Blog and was presented with an ‘Invalid Integration’ error.
I noticed this happen soon after the automatic WordPress upgrade from 3.8 to 3.8.1 so I am suspecting the upgrade as the cause but have no evidence to back that up.
Luckily there is a very easy fix to this problem. Simply FTP (or whatever protocol you desire) in to your server and locate the blog/wp-content/plugins folder and rename the duo-wordpress folder to something else (like duo-wordpress-old)
You should now be able to log-in.
Now, rename the folder back to what it was before and on the WordPress web interface,uninstall the plug-in completely.
Unfortunately, as of this post (28/1/2014), re-installing & integrating the plug-in does not actually fix the issue so you may need to live without 2FA for a while (or alternatively, use another provider)
The problem may be something specific to my set-up but just in case others are having the same/similar issues, I hope this helps you!