My 98 Volvo has the same problem. I don't mean to brag but I'm a mechanically minded and capable of doing my own work. I took the dooe apart and found a plastic piece broken on the lock mechanism. Just one little piece that can onlly be replaced by buying the whole mechanism. $130 dollars from the dealer. I thought that was to much so I've been unlocking it manually. It locks ok just won't unlock. About a year now and I'm about ready to spend the $130. i approve this answer ...very well put
Check out http://volvospeed.com/Repair/v70latch2.php . If you can't find enough pieces to repair the part, Erie Vovo (yes, Vovo) has quoted $75 for a used latch assy. with a lifetime warranty. I pieced mine together with crazy glue and drilled holes into which I put more crazy glue and lengths of steel wire.