@rissykitten and @djy444 Just to clarify, SimGuruMonkey isn't a person, it's the name attached to an automated messaging system. Whenever you register content, you're supposed to get this kind of notification. There was no one on the EA end actually fixing something; the system just worked as intended, eventually.
Community ManagerHi @itsBlurry01 & @djy444
Are you still unable to get your game registered?
Yeah, still unable to do it: "The serial code you entered is invalid. Please enter a valid serial code and re-submit." My case: Steam and EA accounts already linked, serial number the same, email the same. I probably clicked Create an account! in the top-left corner on thesims3 website by mistake, and went through the EA login window by clicking the Steam icon. So it seemed that I created a new account, not the Steam-EA-linked one, which led to the problem.
