Sucks to be you ... (hey, don't hit me ... I've been there!) <G>
Something else you could try ... I just checked and there's an update available on the "LATEST, MIGHT BE LESS STABLE" channel under HELP/UPDATE CHANNELS. Click that so it's tagged, and CHECK FOR UPDATES. Hey, couldn't hurt, right?
Where's all the gurus? We need a guru here!! <tap tap tap>
Another brainfart here ... have you tried opening one of your MKV files using FILE/OPEN MEDIA FILE? That could also force the issue and make it right, or at least confirm that MC has the proper codec and can play the file. Also, maybe a firewall issue? You could try setting that to minimum (your call ... feeling lucky?) and try another re-install. As mentioned, my MC18 install had issues too, and they seemed to be server related at the download site.
Something else you could try ... you don't necessarily have to be on the bleeding edge of up to date. I was playing MKVs on MC17 for quite a while, and the install went without a hitch on that version. Your license should work with all the older versions (back to MC14 if I remember right). You might even have a couple or three of the old installs laying about if you upgraded from a prior version.
Be sure to post back with a fix if you do get it working. Other enquiring minds might need to know.
Luck with it ...