My colleague and I have probably detected a bug in Studio 2014 (brand new version of SDL Trados Studio 2014 Professional - 11.0.3709.0, installed only last week):
We merged segments of an html file in editor (in our case a sentence was segmented after a full stop where it shouldn't have been) and then, at the end, when attempting to export it to a target html file, we got the following error message:
"Failed to save target content: Object reference not set to an instance of an object."
So the problem only seems to arise after merging segments in an html file.
We are experiencing exactly the same problem with SDL Studio 2014 SP1. In our case, we had merged or split segments and used revisions marks.
Were you also using revisions marks in your project? Maybe it is a combination of both.