I run only Cosmic on Fedora 40, kernel-fsync, mesa-git. After updating cosmic-greeter from 28.08 ver to 06.09 it's not starting during boot. Restarting the service doesn't work, gives an error.
Reverting to the previous version fixes the issue.

Don't know if it's only your package, as if I get it correctly it's prepared to be used as is, without any third party DMs (tried on Arch "systemctl enable cosmic-greeter" doesn't run the service) or that's something rooting from the git, thought to report here anyway.