Had this problem elsewhere and it was caused by the Calendar widget which had a file named Calendar.js instead of calendar.js and in 5.15 this, for some reason, caused the login issue (while in 5.13 it did not). Removing the file from the widget directory and importing the latest version from the app store resolved the issue.
So make sure you check Firebug (or if not working in Firefox, your browser's equivalent of developer tools) to see if any widget errors come up, that's how I discovered the source of the issue in this instance.