The Intensity theme is made to work with WordPress 3.5 or newer, PHP 5.6 or newer, and MySQL 5 or newer. However, we recommend you follow the requirements listed for WordPress.
Many issues can be caused by PHP configuration settings that are too low. Issues such as: having problems uploading or installing files, experiencing a white screen when trying to access your site, mysteriously losing menu items when editing large menus, etc. can all be related to PHP configuration settings.
If you are unsure how to change these settings yourself, your hosting company can help. You can see what your current settings are by using the following plugin: