I originally filed that bug in Ikiwiki, but now I starting to think this is a problem specific to the branchable deployment here:
Basically, on ikiwiki.info and git-annex.branchable.com, if I use plain HTTP, it is impossible to login. However, going to HTTPS works fine and I am already logged in.
It is unclear to me why HTTP is still supported if HTTPS just works out of the box: it should just be a redirection and all traffic should be in HTTPS. --
Ah and do note that I am editing this site in HTTP without problem - presumably it's how I logged in here so it works.