I've been getting this same error off and on for a while working with STEP files, and I finally figured out the problem. Inventor can't open files when the combined length of the path and filename is greater than 128 characters (excluding the extension). This behavior appears to be the same on local or network drives. It will happily save STEP files with a longer path+filename length, it just won't open them. I'm using Inventor 2012 SP2 64-bit. This issue doesn't appear to occur with native Inventor files, or with Inventor Fusion at all. Autodesk, can you please fix this bug in Inventor?