I suspect this is a file or directory access issue. Looking for any clues about this. I've installed roundcube 1.6.11, Apache 2.4.62, and tried PHP 8.3 and 8.1, same issues with both.
(https://illuminosi.com/images/roundcube.png)
Fixed it- I needed to change ownership of the whole install, not just under the public_html directory.