Please clarify - how exactly can you not add due dates? You add a new task, go to the due date field and what happens exactly? You can't click on it? It closes? You see an error message? Also - what exact due date are you trying to type in?
I no longer see the a place to put the due date when putting in a new task. I can send an screenshot in an email if you would like.
We've changed the UI a bit and now the task input field has a vertical bar on the right separating the task text field from the date field. If you hit the TAB key, you should get there.
If not, please send a screenshot to firstname.lastname@example.org