We are currently tracking another similar issue like this. Although I can't see how our plugin would cause this, we are looking into this matter further.
Can you please confirm the results of any errors if you uninstall our plugin? Does the issue go away?
Our plugin does NOT 'hack' or rewrite any Joomla core files. the fact that the error is referring to a core joomla file that is missing would be the first thing to look at on your site.
We will see if we can replicate issue on our test site using JLogin (if possible)
I'd love to change the world... but they won't give me the source code!