We have a column in one of our workboards ("6/13") which we can't delete even though there is no tasks in it (even closed):
{F165851 size=full}
{F165853 size=full}
Note that we can't reach the column edit page while in "All Tasks" filter mode because of {T5341}, so I have to click the edit button while in the default "Open Tasks" filter, but I doubt that's related.