I've a strange problem with my Apache server. I secured the Apache's document root directory with the .htaccess and .htpassword. That works fine for five out of six users. Only the sixth user gets this error: Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request. Please contact the server administrators 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. The error log shows me: configuration error: couldn't check user. Check your authn provider!: /
My OS is:Ubuntu Linux 11.04 and my Apache is:Apache/2.2.17 (Ubuntu) PHP/5.3.5-1ubuntu7.11 with Suhosin-Patch configured.

Can anybody here help me to find the clue for this problem?