The problem is that it seems the click-event on the task is not processed correctly.
It should add a task-editing form to the page, but the form isn't found (hence the ArrayIndexOutOfBounds).
I bisected it and the bad commit is
[OPENENGSB-257,OPENENGSB-2816] Upgraded to pax-wicket-1.0.x/wicket-1.5.x
Interestingly the form seems to work correctly when opening the page in the browser.