I've never used tasks.dll but I just had a read of it's docs. Tasks also has !TasksHide and !TasksShow which you could also try. Though I can't imagine why !TaskToggle doesn't work.

You say that only the icons disappear? What about the titles for the applications? Are you sure that the taskbar is not becoming hidden and that another part of your theme which is underneath the taskbar is remaining visiable?

Perhaps you can upload before and after screenshots of your theme into your profile so we can see exactly what effect !TaskToggle is having.