Error message when upload images using KCfinder

I am using version 2.00+ and I have problem when I try to upload images to the server using KCfinder,
I always get error message:

Internal Server Error

The server encountered an internal error or
misconfiguration and was unable to complete
your request.


Please contact the server administrator,
XXXXXXX and inform them of the time the error occurred,
and anything you might have done that may have
caused the error.

Any idea why it happened? Thanks a lot for your help!