[2021] Can't start Stellaris from Steam: Installation Package could not be opened

Please try this:

- exit the Steam app entirely
- uninstall Paradox Launcher v2 from the Windows "Apps and features" utility, if still present.

Delete the following folders if still present:

- C:/users//AppData/Local/Programs/Paradox Interactive/
- C:/users//AppData/Local/Paradox Interactive/
- C:/users//AppData/Roaming/Paradox Interactive/launcher-v2/
To see the AppData folder you'll need to enable View Hidden Items in File Explorer.

- 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//AppData/Local/Paradox Interactive/launcher-v2/
Please attach them here.  

Let's try forcing an update of the launcher.

Download the installer package for Windows: https://forumcontent.paradoxplaza.com/emailsignatures/2020.11-win-prod.zip

Unpack the file you just downloaded and place the three files in the Steam folder for the game you're having issues with. If asked if you want to replace already existing files click "yes" - that's a sign you are in the right place!

Then please repeat the launcher reinstall procedure from before.

Good luck with that!  

It is expected that launcherpath does not exist before installing the launcher. It gets created after the launcher installs. What you are seeing there is just a check by the installer to see whether this is a new installation or not.

That msiexec log stops very quickly, I haven't seen that before, The 1619 error can be that you already have some other installation in progress, or at least Windows thinks you do. In Task Manager-processes do you see MSIEXEC running? If so, kill it. If not, run the Updater app in Windows, does that run okay? Install anything?  

That does read like there is an issue with the Windows updating process, I'm sorry to say! I suspect nothing that uses msiexec will work until that is fixed.

If you have access to Microsoft support I think that is your next stop.

Good luck with that!