I recently found the solution.
You have to go to folder AppData/Local/Uber Entertainment/Planetary Annihilation/
and edit the file boot_setting.json
Change the contents to:

{
"active_content": "BEST",
"content": [
"PAExpansion1"
]
}

The file may be overwritten on next game start. So for safety ...

The source is supposed to be PA-Titans, but instead vanilla PA starts.
Also the mods entry is missing from the main menu (not sure if this is related).
Clicking on "Armory" deliveres a prompt to quit and install Titans (obviously only quits the game).

The game works fine otherwise. Just feels ...

I sunk some time into this. Turns out the crash was linked to nvidia software/drivers settings.
- vsync: "fast"
- preferred refresh rate: "highest available"
Having both enabled crashes the game. Having one of them enabled (didn't write down which), keeps the game stuck in a black screen. And ...

I just tried to test Atrox, and got some weird results.
After install it started fine, but the colors were wrong (e.g. a lot of pink pixels). Sometimes I could make out, what seems to be a profile creation screen. Sometimes it was so bad, I couldn't recognize anything.
I tried compatibility modes ...

just so you know / for your consideration:
4 DLCs have been announced.
the second should be released very soon (fall 2018), and the last will be sometime 2019.
of course, patches after each DLC are to be expected as well

I can open the part38.rar
trying to extract the 02.bin gives a CRC error

not sure if this is any help. since the rar obviously does not contain the full .bin

I am going to download 38, and report.