Despite trying in Chrome and Firefox without privacy protections and extensions, and in incognito mode, 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.