Oh okay, i think i know what this is, Are you using a fullscreen client? Check inside the client files if you are, because i know alot of the fullscreen clients actually pack some kind of cachedownloader into the client.java and i added a cachedownloader into mine, and i started to get this error.
If this is the case, you just need to change the cache links inside of client.java rather than the cachedownloader and REMOVE the cachedownloader from the folder, hopefully that should work for you.
Make a back-up just incase anyway.
++REP if it works