Despite trying in Chrome and Firefox without privacy protections and extensions, I'm getting the fabled error.
You may run into this issue when you use specific browsers or add-ons that hide the referer header for id.atlassian.com and other Atlassian sites. Check for these and then try logging in again.
I see the issue is likely related to a long dormant account and that looks to be changing soon enough for me, so if I could get help around that, I'd really appreciate it.