it is a "task list", but with a custom content type on top.
Because of your question I tried some different things and have new insights:
I created
- a custom list with my custom content type -> form works
- a task list without my custom content type -> form works
- a task list with my custom content type and the form displayed NOT as dialog box -> form works!
- a task list with my custom content type and the form displayed as dialog box -> form does NOT work
So it seems to be a combination of the last settings. But the last few weeks everything worked fine 
Something I noticed while testing:
Last week I created another thread with an untypical behavoir of the displayed form title "DisplayForm".
This problem does not occur in the custom list without a template. Also the form gets displayed as dialog box on the right side of the window:
