Thank you for your help, now it works. I had to try several times, mervis IDE told me that the controller application failed to start. After a while I found the option "Create boot project", that was probably the missing link. Do I have to cold start the controller now every time I change something in my application or my IDE in order to have the working login function?
Further imho the login this way is not a really nice solution, because the whole HMI shown without values and it seems as the web application does not work well. A separate login screen would look much better.