.net 'FredCK.FCKeditorV2' does not match

I am using C# and I get this error :

The located assembly's manifest definition with name 'FredCK.FCKeditorV2' does not match the assembly reference.

[FileLoadException: The located assembly's manifest definition with name 'FredCK.FCKeditorV2' does not match the assembly reference.]

Any help would be greatly appreciated !