My fellow steam users. I also had the same problem. Fixed by doing the following:
1 - Start PSO2 by steam as usual. On the screen that says "Back", just prior to the error window, do not click "Back". Leave the window open and the program running.
2 - Locate your PSO2 installation folder. Generally, it'd be something like "C:\Program Files (x86)\Steam\steamapps\common\PHANTASYSTARONLINE2_NA_STEAM".
3 - Right click on the pso2_bin folder, go to the "Security" tab, "Advanced" options.
4 - On the window that opens, check the box at the very bottom, "Substitute all permission entries of objects" or something. Click apply.
5 - Wait for it to end. It might take a while, it's a big folder. You might have to click "continue" on the following windows several times. Don't worry, it happened for me. It won't impact the game.
6 - After it's done, you're halfway through. Close the PSO2 game you had open. Don't open it again until later, you'll have to do the same steps from 4 one more time, now with the game not running in the background.
7 - After it's done for the second time, open the game through steam again. Did it and it's working like magic. Don't ask me why. I can't understand why the heck it works like this. But it worked for me and others. A thank you to Faith Games for doing a youtube tutorial on the issue (it's in portuguese). I'm playing now thanks to him.