Joomla 3.4 introduced certain environment changes that might cause Wall modules to fail on some host systems after updating to version 3.4. To fix the problem, simply download and reinstall any "Wall" modules like "Deluxe News Pro", "K2 Content Wall", "Socializer Wall" etc. that are currently installed on your website. Reinstalling the modules won't affect your current settings.
You can find download links for them on their product pages or in the downloads section of your template.