I had a similar error and even though I had been working fine for a while, if I clicked on my effects or particles they could not find the source files, but they were there. You might test that. They previewed ok, but when I went to burn got a similar msg. I finally reinstalled, make sure you install Quicktime. That still didn't fix me, it wasn't until I reapplied the latest patch did it fix it.
It also might indicate a broken codex or missing one or lack of memory to process an effect. The fact that it says Media Source Error, points to either the input file and PD's recognizing it properly. The source may not be your video, but could be a particle/effect/title/menu. Knowing how far into the video it stopped can give a clue. First 21% is often menu generation.
Windows 7 Ultimate 64bit
Velocity Micro
Intel Core 2 CPU 6700 @ 2.66GHz
RAM 6GB
NVIDIA GeForce 8800 GTS v258.96
ViewSonic VX2255wm-4 dual extended display
K-Lite 32bit & 64bit
1680x1050
Windows 7 Basic background no Aero
Hitachi 400gb system dr