Studio/ Player Error

it's not /content/generatedUiConf/23/448/ui_conf_23448682__12.xml it should be /opt/kaltura/web/content/generatedUiConf/23/448/ui_conf_23448682__12.xml, the conf_file_path is relative to /opt/kaltura/web.

What does:

ls -al /opt/kaltura/web/content/generatedUiConf/23/448/ui_conf_23448682*

return?
also, easiest way to test permissions is to temporarily edit /etc/passwd so that the Apache user can start a shell session.
Before you read on, be sure to change this back after testing as this poses a security risk if left this way.

apache:x:48:48:Apache:/var/www:/sbin/nologin

becomes:

apache:x:48:48:Apache:/var/www:/bin/sh

this will allow you to start a shell as the apache user, like so:

# su - apache

Then you can check whether you're able to create a file under /opt/kaltura/web/content/generatedUiConf/23/448/ with:

$ touch /opt/kaltura/web/content/generatedUiConf/23/448/test

If that doesn't work, you need to understand why, probably wrong permissions to one of the directories along the path, if you do manage to create the file, then we can assume the issue is not with permissions, in which case, you should check for errors in the Apache error log [/opt/kaltura/log/kaltura_apache_errors*.log] and the API logs [/opt/kaltura/log/kaltura_api_v3.log and /opt/kaltura/log/kaltura_prod.log].