404 Error on upload

When I upload through the user interface, I receive a 404 "Internal Server Error" in an alert() box. The file does not upload.

I thought perhaps my target destination does not exist. But I FTP'd a file to the destination folder and that file appeared in the interface. Additionally, I can move files and create folders successfully through the interface.

Full error message:

"An error occurred:

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

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

More information about this error may be available in the server error log.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

"