EBS R12.2 one user cannot login

We are using e-Business suite R12.2. One user only cannot log into the system. If access is attempted using this user's credentials, they are accepted and the last_logon_date is updated in FND_USER, but the screen greys out and the home page is not displayed. This happens every time for this particular user, all other users are not affected. The url changes (sometimes!) to https://<server_name>:<port>/OA_HTML/AppsLocalLogin.jsp?langCode=US&_logoutRedirect=y

has anyone come across this before? It seems as though the system thinks that there is already a session on progress that is blocking this user.