The only thing I can think of that Glynor mentioned was .NET which I had to install because of Hyperspin (Arcade front end)
There is some component of .NET that is required for some part of MC, but I don't remember the details. I know Hendrik posted an explanation once, but I can't find it. Either way, it does prompt you to install it during the initial install if it isn't there and is required, and it certainly does NOT conflict with any version of .NET that I know about. I have a bunch of versions of it on all of my machines with MC.
Have you tried a full uninstall, reboot, reinstall of MC? That might be worthwhile at this point. Make a Library Backup first, so you can get your crap back when you're done, and then remove MC including all settings (you can leave your Library). Reboot, and then reinstall using the latest build.
Hopefully it is just that something went wrong in the initial install, and that'll fix it. Long shot, really, but worth trying since we're otherwise stumped.