upstream sent invalid status "40" while reading response header from upstream

Hi

When i try to use this link on my forum:

http://www.domain.com/index.php?app=members&module=messaging&section=send&do=form

I am getting 502 bad gateway error but only for this link as all the rest on my forum is working great..

Checking the logs i got this:

upstream sent invalid status "40" while reading response header from upstream

On the net here is no any related info about this error code :(

Any ideas?

I am using Nginx and Phpfpm....

Thanks