Hummingbird minification not working, 504 errors

Hi, I have some problems with Hummingbird. I installed it in a lot of websites without problems, but I can’t figure out why it’s not working on this one.

I installed WPMU Dashboard and then Hummingbird. When in Hummingbird Dashboard, if I click on “Minify my website”, I get a “504 Gateway Time-out” error. This is what I see in the log:

24770#0: *4146472 upstream timed out (110: Connection timed out) while reading response header from upstream
mod_fcgid: stderr: PHP Fatal error: Maximum execution time of 60 seconds exceeded in /path/to/website/wp-includes/cache.php on line 674, referer: http://www.mywebsite.com/wp-admin/admin.php?page=wphb

The same error occurs if I try to disable Hummingbird from “Plugins” or if I click on “Clear”, in the Hummingbird Dashboard, to reset the settings.

I removed Hummingbird by renaming the plugin’s folder, I also removed WPMU Dashboard and reinstalled from the beginning: same error.

I disabled most of the plugins (I left plugins I’ve used on a lot of other websites where Hummingbird is installed and it works without problems) and switched to the default theme, but the problem still remains.

WP_Debug shows no errors and my .htaccess file is clean (just WordPress lines).

Browser Caching and GZIP Compressions checks work fine.

What should I do? Thanks!