How to debug an error 500 in buttonhandler.php

Hi.

I am getting this error: POST XHR https://... /investigadores/buttonhandler.php [HTTP/1.1 500 Internal Server Error 303ms]

I don't know how to find the problem withing the code.

The button only has this in client after event, the rest are empty:

location.href = "menu.php";
return;

Can you advise some pointers to that?

Thanks in advance,

Image:

http://ims.com.mx/AOM/phpr-buttonhandler.jpg