Login issues for the same SQL login on mirror server working fine on principal server
If logins are getting locked, that indicates the password is incorrect.  Either the server has the incorrect password recorded for the login, or the client is presenting the wrong password.  

Since this is a mirroring environment, I'd bet the client is using the same password for both instances; meaning the password for the login on the mirror is not the same as the password for the login on the master.

Recreate the login on the mirror server with the known-good password, i.e. not the hashed version.

Ensure you get the sid correct (use copy-and-paste) from this query, ran against the principal server:

SELECT sp.name
    , sp.sid
FROM sys.server_principals sp
WHERE sp.name = 'login_name';


Use that sid in the CREATE LOGIN statement you run on the mirror:

CREATE LOGIN login_name
WITH PASSWORD = 'thepassword'
    , SID = 0x000000000000 --put the real sid here.
    , DEFAULT_DATABASE = tempdb;


The sid is only important because it links the login to the user inside the database.