Blender not responding when opening file with Rigify

Blender version I’m using: v2.77

I got a FBX file (a character) that I converted to FBX 2013 (using the Autodesk FBX converter)
so that I could use it on this Blender version.
I rigged (with rigify) and animated this character for a company project and saved the file (I decided to create two different saves, as precaution).

Now that I am trying to open it as double-click, Blender opens and goes “not responding” and never actually responds.
Opening it from inside Blender gives the same results. I found a “work around” that if I uncheck the “Load UI” box when opening the file, it works fine.
However, I’m afraid this will be a reason for the company to complain (they might use it on Unity).

Does anyone have any clue on how to get it working and opening normally?