The current pre-release of the client ("pioneer" in the version) is only compatible to itself. So you can only play with other testers. Please be aware!
AccessDeniedException looks like you have your .exe setup to be run as an administrator and the fafclient is not running as an administrator so it doesnt have the permission to start the .exe
Try and go into the .exe settings go to compatibility and remove the tick from the "run as administrator" box