I'm a little confused on your issue, you seems reported two issues:
1. when you login with user "service_desk", system displays the Error "Incorrect username or password".
can you make sure if the username/password is correct? can you login myit server client with this user?
No I mean, when i open application on mobile ,the system auto log in by user "service_desk" and displays the Error "Incorrect username or password".
I think system default "User : service_desk"
2. when you login with user"app admin", there is no error, but the account name displays as "service_desk".
where did you get the myit app? can you try this by using the app under myitserver/IPA/download?