What version Divx are you using?
The old version will not play MKV/MP4 files you have to use the new version or VLC.
IF you have the old version it will show a Flash button instead of Divx.
But the first thing to try is clearing Divx Cache folder.
While the video is trying to load Right Click on the player and goto>preferences>then cache and click the clear cache button.
You should also clear your browser Cache. you didn't say which you use so search or post back if you don't know how.
That should fix your problem
I'll check back in a few but more Info more help (yes) browser,os,player versions