Wiki plugin activated >> Internal Server Error

Hello,

As soon as I try to activate the Wiki plugin, I get the following message :
Internal Server Error

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

Please contact the server administrator, postmaster@www.oreille-malade.com 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.

Any idea ?
Best Regards,
Laurent