Missing e is not the cause of recent account trouble some people are experiencing.
When logging in, some Missing e users are seeing the above screen, but it is also happening to people not using Missing e.
Sometimes, this can mean that your account has been accessed by someone without permission. Perhaps you have entered your password on a page that was actually intended to look like Tumblr, but was actually a phishing attack where someone uses a fake form to steal your password?
Change your password and keep it secure.
Above all, make sure that you only enter or change your password on a secure Tumblr page. Here is a Tumblr staff post on how to recognize secure Tumblr pages: http://staff.tumblr.com/post/15260097735/ev-cert
As I find out more, it appears that the problem may not be compromised passwords but a bit of a mixup on Tumblr’s end.
That link to a Tumblr tweet may point to removed tweet. Tumblr has since tweeted about this exact issue: https://twitter.com/tumblr/status/212702358604480513