Malformed Secret Error on Authentication

i'm working on kentico verison 12 sp for mvc and we implemented authentication handler to send OTP to user by email the pass code is sent on local machine but on server it gives an error Source:MF Authentication Event code:MALFORMEDSECRET User '' has a malformed MF secret. Message: Error occurred during a cryptographic operation. Exception type: System.Security.Cryptography.CryptographicException i've searched and added machine key on server on webconfig but still it gives the error and not able to send key what is the solution for this errori found the it gives error for previously used users, but new login is working fine !!