Answer by Basheer is correct. It is technical error.
I have one more opinion that it could be because of Aadhar service is not working now.
In the particular linked question you can check that while issuing DSC, Aadhar based OTP is not working.
I doubt this could be reason we are not able to file return with EVC generated.
I think it should work with DSC. Anyone tried filing with DSC?
chatCommentsSir, I have confirmed by the GST officials and it is technical error and Kindly find the update regarding "Error! Authorized Signatory details not found or user not registered" http://magnetinfo1.blogspot.in/2018/01/update-regarding-signatory-details-not.H T M L - Jalees AhmedPlease stop pasting link. You can use the below answer box (bottom of page) to answer question. Do not paste link, don't you think if users can get answer here itself. - Pulkit Sharma