Hello Andreas,
Are you sure that you performed all the steps that must be performed during an upgrade? For a list of the necessary steps, see Upgrade to New Version. In particular, if you have custom Web Interface sites created in addition to the default 3 sites, pay attention to the 5th step that requires you to update the files in the folder for the custom Web Interface with files from one of the existing Web Interfaces (for Administrators, Helpdesk or Self-Service), and then reconfigure the custom Web Interface.