Ran into this locally - I was using `drush uli` to jump into a local copy of the site. Just for context, I was using Docksal locally, my site was fully functional on Pantheon, no issues using `drush uli`, but `fin drush uli` (Docksal's implementation to run a drush command on my container) on my locally was sending me to an "Access denied" page - I cleared cookies for my local site, in this case `mysite.docksal.site`, and voila! I am back online. I understand this probably won't help a ton with an actual fix, but my primary hope here is to aid any future devs that might have found their way here, and offer a workaround solution.
Confirmed this fixed our integration with MIMEMail - wasn't happening with SMTP formatter, which had us scratching our heads, and appreciate this thread and discussion - patch worked as expected, thank you!
Hey Rajab - I pushed the patch to 8.x-1.x - Let me know how it's going, thanks for the patch.