I'm surprised someone from Telerik hasn't answered this question from October yet.
I was also completely stumped on this issue until today. Setting any page with the login widget (Sitefinity v. 5.2, current widget version, using Claims authentication) to require SSL would result in "error:" and no explanation when the form was submitted.
This obscure setting, in the regular Sitefinity documentation, worked for me:
"If you are experiencing problems with the login when an SSL certificate is installed on the login page, click Administration » Settings » Advanced » Security. In AccessControlAllowOrigin, enter *. If this does not solve your problem, return its value to null."
** edit **
The fix above worked for FireFox, but NOT for IE (ver. 9), which comes back with "error: undefined".
Can someone from Telerik chime in? Not being able to encrypt login pages is a MAJOR security issue, and prevents PCI compliance.