The error message occurs because you are trying to load XVid video onto the timeline then MEP cannot find the codecs because they are either corrupt or missing.
Have you uninstalled XVid?
Download and reinstall the XVid codec/software - only use the genuine software.
xvidcore.dll is a part of XviD codec, which is required to play the XviD movies on your system. With it, you can play your XviD movie and video files using any media player that currently exists on your computer.
If the XviD codec is not installed on your computer, your XviD movie or video files can’t be started properly. In this case, to fix the error, you will have to download the latest version of the XviD codec on your computer from the XviD website and then run your XviD file.
If you already have XviD codec installed on the PC, make sure it’s compatible with your operating system.