Hey Chlup,
Weird problem indeed. Might be a synchonization issue between the server and client. If you are still having this problem and are willing to experiment try to logout (really logout via the logout link on the homepage) and stay logged out for at least 15 minutes. This ensures that your active session will be terminated server side. After logging in again, give the game 5 minutes to load before you do things.
Let me know if it worked, if it didn't, the only thing I can suggest is to contact Support via the contact form on this page: http://www.thesettlersonline.com/en/help-and-support