Installer fails to install "important update"

From package testing Qt 6.5-rc.

With the current installer on my system, I could not download Qt 6.5-rc package, since the installer told me that an important update needed to be installed first. But from the list of packaged, I was not allowed to select and install that package (or anything else). So I had to download the latest version from qt.io. Then I was able to install Qt itself. But when trying to start the Installer again later to install an extra package, the same happened once more; I had to install "an important update" first, but was unable to select it in the UI. This time it also had an error message on the right side of the dialog that told me that "there was an error downloading the selected component. See screenshots.