After failing over to a secondary AlwaysOn replica, you may find that you are unable to access certain tabs in DataNow Admin Console, such as Active Directory. Clients return an HTTP 500 error.
SQL logs show invalid credential warnings for the DataNow user account on the replica instance
This can occur if the SQL logon account was configured manually on the replica.
This results in diferent SID (Security identifier) values for the DataNow user account which results in the symptoms above
Refer to https://support.microsoft.com/en-us/kb/918992 for the correct method of transferring the DataNow user account from the primary instance to the replica instance applicable to the version of SQL server that is in use