A tatamibari solvability question
Tatamibari is a game published by Japanese game company, Nikoli. We are given a grid and some of the squares contain a symbol; either + or  or . The player must partition the grid with line segments drawn on grid lines so that
 Each partition contains exactly one symbol.
 Every partition is rectangular.

update: I left this condition out. No four rectangular regions may meet
(this is the tatami condition!)
 Partitions with a + are square shaped.
 Partitions with a  are wider than they are tall.

Partitions with a are taller than they are wide.
Here is an example puzzle and solution:
Fun puzzle, right?
Here is the real question: How do you generate puzzles with solutions?
Take this one, for example, which obviously has no solution.
So we can’t just scatter the symbols onto the grid randomly. As far as I know this problem is open:
What are the necessary and sufficient conditions for solvability of one of these puzzles?
And this one might be open too:
What is the hardness of finding a solution?
update: I should point out that one can always construct a puzzle by partitioning into rectangles and inserting the appropriate symbols.