Is your server on PHP 5.3+ ? If not, you need to upgrade to latest version of PHP. Check PHP version with http://wordpress.org/plugins/health-check/
Otherwise this sounds like a plugin/theme conflict.
To test for a theme conflict:
- Activate default Twenty Ten, Eleven, Twelve, or Thirteen theme
- Check to see if the issue still occurs
- If the issue does not occur after having activated the default "Twenty" theme, your theme is causing a conflict with landing pages.
To test for a plugin conflict:
- Deactivate ALL plugins
- Activate Landing Pages
- Check to see if the issue occurs
If the issue does not occur, one (or more) of your plugins is causing a conflict with Landing Pages.
To determine which plugin(s) is causing the conflict, follow these steps:
- Activate each plugin one by one
- Check to see if the issue occurs after each plugin is activated