Fast User Switching to lock computer crashes login screen

[lhunt]

I have a user when he goes to lock his computer using Fast User Switching (he does not want to log out) can not get logged back in. I've also tried to log in as a local admin account, and can not do so. If he logs out of his account he has no issues logging back in. When I have him disconnect the DisplayLink before going up to click on FUS menu he has no issues getting logged back in.

He's on a MacBook Pro 2011
OS X 10.9.2
DisplayLink v 2.1 (i've also tried beta v2.2)