I've just done some more debugging and you can reproduce it by first using the wrong login details and then logginging in. The first attempt posts login data to the /<user>/login page, but subsequent attempts go to /<user>/setlist I've just tried the correct details after a page reload and it works! Has to be the first attempt apparently.
As per the title, I am not able to sync sets to brickset from "my sets" page. I log in with my correct details and get two errors If I use my actual username and made up password I get both errors. If I use a made up username/password I only get one error Chrome on linux.