Upload form not displayed

Hi.
I ran a photography contest using your plugin one year ago and everything was perfect. I was thinking about running a new one but I cannot see the upload form using the short code.
I have changed my web server a few weeks ago. Before I was using apache + mod_php and now I am using nginx + php-fpm. I think that the problem could be related with this change, but the gallery of last year contest is working ok, so I don’t know what is going on.
It would be great if you can give me a hint about what can I do, I’ve been looking at nginx log files but I didn’t found anything.
Many thanks!

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