Error 404 when activating plugin

Hi there,
I have a problem with the plugin, I developed my website locally with MAMP (PHP 5.4) and it worked perfectly, however, when I migrated it to the server (hostgator), I got the error saying it requires PHP 5.3 (it had 5.2), so I activated 5.3 and now, when I click “Activate” I get redirected to a 404 error page.
I am using a custom theme but I have also tried to use the default ones (twenty twelve) with no success.
I have also checked for the mcrypt library and it is enabled on my host.
Any ideas on how to resolve this?

PS. when I revert to PHP 5.2 I do get the checklist of requirements and all of them appear correct except for PHP version.

Thank you very much!

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