I believe that the issue here stems from the fact that Technic bricks and System bricks are not 100% compatible. Here's a slide from a great presentation which explains the details:
This difference is barely noticable when building, especially in your scenario because the hinge adds some additional play, but I believe that LDD won't allow these to connect because they are actually off by .12mm.
I tried to take a few pictures to demonstrate what I mean, but they didn't turn out very well. Here's a System brick (tan) connected to a Technic brick (blue). You can just barely see the .12mm difference at the top:
The difference becomes more obvious when more bricks are added so that the error is allowed to compound: