I had problems when launching the hooked game, which I actually solved. So I am creating this topic for people, that may run into simillar problems like I did.
I have the Steam version of Red Alert 3 on Windows 10. When I hooked it with C&C Online, and tried to launch it from the C&C Online launcher, I got the "Application load error 5:0000065434" in Steam, or the Red Alert 3 splash screen would show, but the game would not start.
In my case it was due to the fact that my steam was running under my user permissions. It was a simple mistake, but it was hard to find from theese non descriptive error messages.
So my advice for all that run to problems while launching the hooked Red Alert 3 from Steam, is to make sure that Steam is actually running under administrator permissions (by right clicking on steam -> Run as administrator). And then simply launch the game directly from Steam. Running C&C Online launcher as administrator was not enough for me, and this helped.
Hope this will help someone 
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