This error seems to be caused by a lot of requests being sent at once (i.e adding something to favourites quickly). I can reproduce this behaviour by clicking the favourite button again and again, however after a certain period of time I am allowed to favourite items again.
Weird. I tried it on two different devices on the same network. At the one device (Windows 10) it did not work and on my other device (Chromebook) it worked. Now it does work on both devices by the way.
Does your network that you are using have any active blockages?
The “429” indicates that you are being throttled because too many favoriting and unfavoriting requests have been coming from your account, but this shouldn’t trigger unless you try to favorite or unfavorite more than 1000 games in a minute OR your IP is blacklisted for some reason. We will look into whether something else may be going wrong that might cause this.