Synchronization can sometimes take a long amount of time in order to compute the items that require update. When running behind a reverse proxy, the request can time out.

The solution is to increase the timeout at proxy level.

## Nginx

Add the following to the configuration file:

    proxy_connect_timeout 300;
    proxy_send_timeout 300;
    proxy_read_timeout 300;
    send_timeout 300;

And restart the server.

See Nginx Proxy Setup for more information about the Nginx setup.

If it still doesn't work, try increasing the timeout.