Exchange Upgrade 2019 CU9 to CU11

After looking at support tickets, I do not think this is directly related to an issue with a CU. In other words - we got no tickets like this that would indicate that something in the CU is causing this. I have found an older support ticket indicating that there might have been an issue with a web.config file, though, and replacing the file has sorted this problem out. But please make sure that you back up the current version and also - I have no way of knowing if you have any custom settings in the web.config that would need to be transferred over.
If still stuck, please open a ticket with support as that would probably need to be next.