PHP Fatal error: Cannot declare class Base_MixpanelBase, because the name is already in use in wp-content/plugins/wp-smushit/vendor/mixpanel/mixpanel-php/lib/Base/MixpanelBase.php on line 8
Found the issue, the class "Base_MixpanelBase" is being used within another plugin "wordpress-23-related-posts-plugin" Once I found the second instance of "Base_MixpanelBase" in that plugin. I disabled it and the site came up fine. Maybe change the name of the class to something else? I think other people would have this same issue if they have these two plugins installed together:
wordpress-23-releated-posts-plugin
wp-smushit