Problems on my site (unknown what)

Hello !

I did a scan on my site via Securi and got the next information:

“Site error detected. Details: http://labs.sucuri.net/db/malware/500-error?v1
HTTP/1.1 500 Internal Server Error”

Is there a way to repair this ?

Thank you for your help !

Jan

P.S. Probably this has happens through “Fancybox”!??