I've run into this a couple times, where I try to log in with my services username and password and it gives me the error below. I'm fairly confident that both username and password are correct in these cases. When I return to the authentication page and refresh with an 'F5' it allows me to log in. It might be that a cookie is expiring?
Timestamp: 2015-11-18 08:16:30.530
Message: Invalid direct reference to form login page
Status code: 400
User agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0