Moved to Tech Support  

Please try this: - uninstall Paradox Launcher v2 from the Windows "Apps and features" utility, if still present. Delete the following folders if still present: - C:/users/{userName}/AppData/Local/Programs/Paradox Interactive/
- C:/users/{userName}/AppData/Local/Paradox Interactive/
- C:/users/{userName}/AppData/Roaming/Paradox Interactive/launcher-v2/
To see the AppData folder you'll need to enable View Hidden Items in File Explorer. - exit the Steam app entirely
- run steam.exe as the Windows Admin user
- run the game from the PLAY button in Steam IF that doesn't help, in this folder are some .log files:
C:/users/{userName}/AppData/Local/Paradox Interactive/launcher-v2/
Please attach them here.
To see the AppData folder you'll need to enable View Hidden Items in File Explorer.  

Unusual errors 1619 and 1620, but still. definitely an issue in the Windows installation service msexec.exe

ERROR_INSTALL_PACKAGE_OPEN_FAILED	1619	This installation package couldn't be opened. Verify that the package exists and is accessible, or contact the application vendor to verify that this is a valid Windows Installer package.
ERROR_INSTALL_PACKAGE_INVALID	1620	This installation package couldn't be opened. Contact the application vendor to verify that this is a valid Windows Installer package.

Try deleting the .msi file in the Steam-Stellaris folder, then do a Steam Verify to make it come down again. If that doesn't help, try this one instead:
https://launcher.paradoxinteractive.com/v2/paradox-launcher-installer-windows If you still get the errors, it's a Windows problem I'm afraid! Make a new Windows user on this machine and log into it with that - any better?