Engine Error: Unknown file version (paks\Win64\pc_all.opt.starpak)

Here is my solution:

1-) Create a desktop shortcut (if you didnt)

2-) Change the target of shortcut on desktop from "E:\Origin Games\Apex\r5apex.exe" to "E:\Origin Games\Apex\r5apex.exe" -eac_launcher (My folder located on "E", this could be different for you)

Hope it helps.

By the way, if you didnt create a desktop shortcut, you can just right click on r5apex application file and click on "create a shortcut" then move it to desktop.