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This is a "catch all" error message from WordPress, any problem, that is bad enough not to show the page, will cause this message. It can be a small thing, like the Smart Slider 3 isn't activated, or it can be bigger problems, that some files are missing. To be sure, delete the Smart Slider 3 plugin, install it again, as you see at the Upload through ftp method, just to be sure that all files are in place, and then activate it in the back-end. (Your sliders will stay.)