I got the same error on both my phone as well as my PC. Yesterday morning and evening, and again now. After restarts. I got the same error in Edge. I also tried going to the affiliate sign-up link in incognito mode, (which worked!) and then logging in again, which then gave me the CSRF error.
Last night, I emailed forums at permies. The end result was a helpful Permies Helper told me to post it here.
I searched in Tinkering for errors with the seller link, and I didn't find anything that matches what I'm seeing. (I know that doesn't mean it's not there).