I have a .NET website that connects to a SharePoint online site in order to access content from there. The authentication is done through the Office 365 login. In order to do that, I composed the following URL:
https://login.microsoftonline.com/login.srf?wa=wsignin1.0&rpsnv=3&rver=6.1.6206.0&wp=MBI&wreply=https://www16290.sharepoint.com/_layouts/15/landing.aspx?Source=" + window.location
The wreply parameter varies, depending on the SharePoint site that the user entered. This code used to work perfectly for more than a year to obtain the STS token and then the FedAuth and rtFa cookies.
Since a couple of days ago, it doesn't work anymore. The behaviour is the following: the browser navigates from my website to the login.microsoftonline.com address, it presents the user with the login form and after authenticating against Office 365, it lands on the error page https://www16290.sharepoint.com/_layouts/15/error.aspx
In the developer console I see a 500 Internal Server Error on their end. Our implementation hasn't been touched in over a year, so I'm assuming Microsoft changed something which resulted in breaking our authentication mechanism.
Any help or suggestion is greatly appreciated.