Same problem with MC 14.0.165, Win 7 Ultimate x64.
The Problem is definitely with IE 9 RC. Installed today and got problems right away. After failing twice MC gets around the bug and launches succesfully. The only difference I noticed is, that the "Options" button of last.fm button gets a non TT font, the grid on some lists (Downloads, last.fm) look somehow weird. But the the last.fm functions as axpected.
After closing and new launch, the "2-times error, third time OK", cycle begins once again.
Update: Uninstalling IE 9RC solves the problem. Somehow cumbersome workaround is, to rename the index.html files in the directories X:\Program Files (x86)\J River\Media Center 14\Data to something that IE does not want to process. This has of course as result, that the "Start", "Drives & Devices" etc Entry pages don't show up.
Apart from this, MC works fine. I have to say, I only do audio, no input for pictures & video. I want to use IE9 and MC 14, so for the time being, I think I have to live with it. Some day soon(hopefully) , a patch will be released, and all I have to do is to rename the index.html files.