Had an issue all of a sudden today opening a vb6 project we use all the time. It gave a cryptic message about a file format no longer being supported.
It turned out I had to go delete a corrupted oca file from teh vb6.exe folder. Best thing to do is crack open process explorer to see what file it's blowing up on and then go delete that OCA. Next time you add the reference it will get regenerated and then things are great. Where were ya on this one google?