Error validating user ID message on iPhone

I have traveler installed on the same server as mail. Traveler seems to be running OK. Long time user with an iPhone when he tries to login the message is

The Lotus Notes Traveler server encountered an internal error validating your User ID

Can’t find a reference to the error.

Have he or I missed something?

Subject: Did you manage to solve this?

The error message is misleading, nothing to do with ID file , or User login.

The issue lies with the access right to the mail db for this user, and for the Traveler server.
You may look to the doc about NTS_USER_SESSION . Traveler server must be trusted, and in the ACL, and the user may have access rights to both servers, and so on.

In my case, the issue was very sneaky : I had set an ACL on the /mail directory. And the user was not listed in that Domino ACL.