Only one specific user? Data corrupt? Does the modeler log show anything? Can you delete the user and define him again? Maybe dive into the database to do so?
==> O, maybe you mean the website and not the Mendix app ;-)
Still not able to log on =(
Just to make sure,
you modeled an application, defined Module roles and defined Project roles, and associated you Project roles with the Module roles, and you are error free (as not being error free would not allow you to deploy).
Is your security actually on ?? and if so, is it set to Acceptance or Production ?