IPhone : Error Domain=Parse Code=209 "The operation couldn’t be completed

I installed the ParseStarterProject swift version and created a new user with the following code:

var user = PFUser()
...
user.signUpInBackgroundWithBlock...

now I'm trying to run the app again and login with the following code:

if let currentUser = PFUser.currentUser() {
    println("current user: \(currentUser.username)")
} else {
    PFUser.logInWithUsernameInBackground...
} 

I'm getting the currentUser printed OK and the code does not do anything else, but then I get the following errors:

2015-03-26 09:49:25.180 ParseStarterProject[14768:199515] [Error]: invalid session token (Code: 209, Version: 1.7.0) 2015-03-26 09:49:25.485 ParseStarterProject[14768:199520] [Error]: invalid session token (Code: 209, Version: 1.7.0) 2015-03-26 09:49:25.489 ParseStarterProject[14768:199521] [Error]: Failed to run command eventually with error: Error Domain=Parse Code=209 "The operation couldn’t be completed. (Parse error 209.)" UserInfo=0x7fcda3cb6680 {error=invalid session token, code=209}

what have I done wrong??