This is what worked for me.
PROBLEM: With my Paradox games (HOI4, CK3, Stellaris, etc.) freshly installed in Steam on my new Win11 Pro machine, and clicking PLAY in Steam to start any of the games, the launcher would try to install but threw an error about a temp file not being a valid Win32 application. This prevented the Paradox Launcher v2 from fully installing and the game would not start.
SOLUTION: tl;dr - Set "dowser.exe" to run as administrator only. Start your Paradox game(s) so that the launcher fully installs. Then set dowser.exe back to normal.
In Steam, I picked just one of the games (CK3 in my case, but I don't think it matters) and went to "Manage > Browse local files" and then looked for the "dowser.exe" (which may or may not be buried in a subfolder, depending on the game). In Properties of that file (under Compatibility tab) I set it to "Run this program as an administrator". After clicking Play to start that game (CK3 in my case), the launcher installed and I could play that game and all the others (without needing to change their dowser.exe files). Most of the games still continued to throw some minor errors or access popups related to either Windows protections or Steam access issues. But I went back to the dowser.exe file that I had changed, and cleared the administrator start. After that, all games started without errors, as expected.
PROBLEM: With my Paradox games (HOI4, CK3, Stellaris, etc.) freshly installed in Steam on my new Win11 Pro machine, and clicking PLAY in Steam to start any of the games, the launcher would try to install but threw an error about a temp file not being a valid Win32 application. This prevented the Paradox Launcher v2 from fully installing and the game would not start.
SOLUTION: tl;dr - Set "dowser.exe" to run as administrator only. Start your Paradox game(s) so that the launcher fully installs. Then set dowser.exe back to normal.
In Steam, I picked just one of the games (CK3 in my case, but I don't think it matters) and went to "Manage > Browse local files" and then looked for the "dowser.exe" (which may or may not be buried in a subfolder, depending on the game). In Properties of that file (under Compatibility tab) I set it to "Run this program as an administrator". After clicking Play to start that game (CK3 in my case), the launcher installed and I could play that game and all the others (without needing to change their dowser.exe files). Most of the games still continued to throw some minor errors or access popups related to either Windows protections or Steam access issues. But I went back to the dowser.exe file that I had changed, and cleared the administrator start. After that, all games started without errors, as expected.
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