macOS suggests update to wrong version

I am currently running macOS Catalina 10.15.7 and want to upgrade my system, eventually to monterey.

However, the macOS softwareupdate program cannot install any update.

Softwareupdate suggests to install macOS Catalina 10.15.6, which is one version below my current version. But this installation fails. I also was not able to manually install a security update.

To solve the issue, I deleted all update-folders under /Library/Updates, but this did not solve my problem.