Uninstall the beta 4.99.4 version you installed from the Ubuntu repository and install the official version using the command line from
https://calibre-ebook.com/download_linux . If and only if, you still have the same issues, feel free to report them in these forums.
And note the thread referred to was
Plugin devs: Upcoming migration to Qt 6. This is discussing the upgrade to calibre 6 and it's upgrade to Qt6. No one is discussing issues with running a pre-calibre 5 beta.