I hope you fixed the problem!
Kory Kramer here posted a link with all the needed solutions. In our case, we had the latest NET framework installed, yet I was affected but my colleague was not. I installed the latest patch (2.9.5) and all works well now. I tend to wait till the new version makes at least a couple of updates before switching. Arc GIS is a sensitive thing, and my projects are too complex to having to fix them if anything goes wrong 🙂