Sub-Task Create Window not shown

[Marco]

We got the problem that it is not possible to create a subtask for a Task. 

As you can see i can see the Button for creating a subtask but if i click it only this line appears 

When i click on Erstellen (create) nothing happens. In Other projects the create window appears like it should.

i've compared the settings for both projects side by side but i could not figure out any difference 

here is also a screenshot of the configured screens 

It looks exactly like in the other projects. What am I missing here? 

[Marco · Accepted solution]

Found the Solution:

The project had an assigned required field that did not show up in any screen. Deleted that field since it's not needed anyway and finally subtasks can be created.