It does look like its hitting the crown guard to me .
The first thing i would do would be to remove the crown and stem, then remove the crown from the stem, then see if the crown fits proper with out the stem in place.
If it doesnt fit OK without the stem, try to see if the crown is contacting the crown guard.
You might just need to file the inside of the crown guard a little.
If its not contacting the crown guard, the tube might be out to far, like you said, so you could either file the tube gently, or maybe get a deeper crown, like off a seadweller.
If it fits OK without the stem, you might try to do what SSteel suggests.
If that doesnt work, you might need to shorten the stem a bit.
EDIT: of course it could also be bad threads on the crown or tube, youll have to be the judge if that when inspecting it.