Is it possible to create a task with multiple lines. If I drop enter, then the task is closed. Is there a button to create a new line?
Unfortunately, that's not possible, it was a deprecated feature. If you wish to add long text to your task, please use task notes instead which allow line breaking and no character limitations.
There is a workaround Guido. On a Android-tablet the line break works fine. If needed, I edit the text on my tablet.