I helped out a someone who had setup their own MU site and wasn't able to login to WordPress. Upon entering login details the page just reloaded. If the password was incorrect, it would show an error.
I solved it by disabling this plugin, I'm not sure how they had it configured, or if there may have been another plugin at play that'd be causing this?
None-the-less, I thought I'd just flag it here, incase there was an issue or if you'd experienced this before.