Edited: ignore this post, please, read my comment below.
Thanks for trying that, sorry to hear it didn't work.
To try to find out what's going on, knowing if this is an issue only in thesims3.com or in general would be very useful. So let me ask a few more questions:
1) Was this the first time you logged into thesims3.com? If it was the first time, did you create an account before logging in?: https://www.thesims3.com/register.html
2) If you follow these steps, do you get the same error?:
Close any incognito windows you have open, and open a new incognito window in your browser
Visit forums.thesims.com
Try to sign in
If you do get the same error, could you close that incognito window, open a new one, and let me know if you can log into origin.com without any problems?
@Theta-3119 Unfortunately, redeeming the Dragon Valley code wouldn't be enough, as you would still need to log into thesims3.com to download the world - so let's try to find out what's going wrong there instead.
Thank you!
-Mai