What is the URL being rendered on the page? I've found a setting change in the upgrade which needs to be reverted back or updated in some cases.
Go to Settings>System>Output Filter and make sure the Exclude XHTML filter URLs is set to " / " globally and that the site inherits it. Not saying this will fix the issue but this is one thing we see consistently with the upgrade from 8 to 9 that helps. Be sure to clear cache and restart the website after you make this change.