Hey Doron,
thanks for your hint! 
I corrected that and i also saw that i used the submit-function on my login button too. So one of these things caused the problem. I chose Data-binding for my further workings.
While we´re at it: I desperately looked for an example code that queries trough my database and checks if it contains the exact email and password that were typed in into my login lightbox.
So basically i am looking for a query-function which gives out an error message, if the mail and password are not in my database. I tried to insert something on my own before but it didnt work.
Any ideas?