Recover Username and Password not working

HI,

A strange occurrence: the ability to recover username and password works in Chrome, but not in any other browser.

In Firefox we get:

Not Acceptable

An appropriate representation of the requested resource /index.php could not be found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

In all others, except Chrome, it is a very similar 406 error message.

Can anyone assist in letting me know what is happening and how to fix it? It would perfectly on my testing server...