After the update which claims to be successful, when I click Nextcloud service’s “launch UI”, I am asked to open or save “index.php”. What does this mean?
Try going to the Nextcloud service page in your server’s UI, click on “Config” and click on “Save”.
If after that you are still running into issues, we would need to know the output of the service health checks as well as any errors showing up in the Nextcloud service logs.