I am having the exact same problem. Although downgrading RPH didn't work in my case. As soon as I click RPH, it starts thinking, and it even pops up in the task manager, the same as Pav said, but then just disappears.
My retail GTA5 starts no problem. I'm just unable to start RPH. Also, not sure if it's a connected issue, but when I do an install of LSPDFR, at the end of the installation when you get the option for the "First Time Run Configurator", I have that checked to open, but it doesn't either.
I have to say, this is a brand new computer, fresh install everything. built it last week. There is no antivirus- firewall has the proper exceptions. Smart screen is all the way off. I'm 99% sure that I got all the proper required programs. And I know for a fact that this computer will run it. (1080, i7-7700k, W10Home, 16gbRAM).
It feels as though I've exhausted all the "fixes" that are posted everywhere on the net. Like the big long one on the RPH forums, and even the Creators update fixes didn't work.
I even tried taking an existing WORKING copy from my other rig, and copying it over to this new one, and still nothing. There's not even a log file to show you.
Any help at all would be greatly appreciated.
Cheers!