Internal Server Error + Additionally, a 500 error

Buanas a tod@s
Estoy intentando configurar una pagina 404, he creado un artivulo 404 y luego un item de menu lara el 404 y cuando intento roducir el error modificando la url para que salte me da un error 500

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

Please contact the server administrator, webmaster 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 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.
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No me da más informacion del archivo donde se produce

S2