I installed Under construction and activated the plugin. I also enabled it in the under construction settings. When I go to my website http://www.brick-design.nl it still doesn’t work.
You could try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
Also make sure you have X WordPress and Cornerstone updated to the latest versions according to :