OSX - Sync API - [ERROR] unable to verify link request

Coded in Swift I implemented after the Tutorial.
DBAccountManager is setup in AppDelegate on applicationDidFinishLaunching

Later, when the user activates dropbox support in my aplication I'm trying to link the account.
The Window Panel is displayed and my application is waiting for the callback.
Sometimes I do not get a linked account, even the user logs in and accepts.
The Log says "[ERROR] unable to verify link request".
When this occurs on a machine it wont't work, you can retry and retry...
if it worked, it works like a charm and in future I always get the linked account directly from the library without the login window.

What does this error mean and what can I do?