Copying from the cross post:

Update: I managed to find and fix my issue. Turns out PSCX2 thought I wasn’t logged in, even though I was. So I logged out and back in again and it sorted itself.

I found it out by opening PSCX2 directly, going to Tools > enable log window. Then I launched the game and looked at the log window and saw a section starting with the word Achievements, and it said something along the lines of “skipping check as we’re not logged in”.

Hopefully this will be some help to someone in the future!