FWIW had same issue this afternoon on two different machines, neither had logged into site before. One Win 7 + Chrome/IE. other a new untainted laptop with win 8.1 + Chrome/IE with no previous cookies saved. Both had same issue as Op. Both I reset browser cache to day zero, but same problem.
I eventually got in by unticking remember me and changing focus to username field and hitting enter.
Now however it works as designed so all good.