OK, I've found the problem. When I entered the dialog to input the estimated taxes paid, the dates were already filled in. I didn't pay attention to that and wrongly assumed they were correct for each quarter.
Virginia's payments are due the 15th of April , June, and September of the current year, and the 15th of January the following year.
TT prefilled the boxes with July 15th, July 15th (again), September 15th, and January 15th.
Once I corrected this, my "underpayment" disappeared.
(note: to change the values it is done in the Federal tax section)
