Usually means Joomla menu item access got mangled during the migration. Edit your Joomla menu items to CB, ensure they've the proper menu types, ensure they've proper access and re-save them. Then see if issue persists. If issue persists turn off Joomla SEO and check what the Itemid is in the URL to find what menu item it's trying to use then fix the settings on that menu item.
Additionally be sure CBs system plugin in System > Extensions > Plugins is published and public as it's mandatory for CB to function properly.