I'm late to the party, but for me this usually indicates there is something wrong with the META-INF files. You can just swap them out for a set from an earlier or later build of the same rom and generally that takes care of it since those files are generally old and not updated frequently.
In tihs particular case, I think I do remember one recent Sultan build missing Cyanogenmod files in the META-INF folder.
There's me jumping the gun and missing that the original poster was using an old version of 4ext. I'll leave the info here anyway in case it helps someone else.