you can try this if it's related to a previously existing game setting :
1 In Steam, Right click on the "Homeworld 3" app and select Properties
2 You'll see a "Steam Cloud" option, (should be the 2nd one), turn it off
3 In your Windows Search bar, (in your task bar at the bottom of your screen), type "run" and press Enter, you should see the "Run" prompt
4 Type "appdata" in the text field and press OK, this will open the AppData folder
5 Go in "Local", then "Homeworld3", then "Saved", then "SaveGames", you'll see a few files in there, find the one named "SavedGameSettings.sav"
6 Move that file somewhere safe so as not to lose it, i suggest your desktop. The goal is for the "SavedGameSettings.sav" file to not be in your "SaveGames" folder anymore.
7 Try to launch the game again and tell me if it works or not
If it fixed your problem :
Make sure to check if no important save game data was lost just to be sure :
- In the Main Menu, click the "Campaign" tab
- Make sure you still see all the missions you've completed as unlocked
- from within a multiplayer lobby, check on the top right to make sure you still have your War Games level (if you played before)
The thing that will be for sure reset is your game settings and preferences, so if you had setup specific things, you'll need to set it up again.
If everything looks good, then do the following :
- Go back in Steam and right click on the "Homeworld 3" app and select Properties
- Turn the "Steam Cloud" option back on