SAP Message XT028 - User &1 could not be checked in system &2

Diagnosis
The Change and Transport System attempted to check that the user &V1&exists in system &V2&. To do this, it uses the function moduleSUSR_RFC_USER_INTERFACE. An error occurred when doing this:
User: &V1&
User type: &V3&
Exception: &V4&
Meaning of the user types:
A Dialog
D BDC
B Background
C CPIC
Meaning of the exceptions:
1 user_dont_exist
2 user_already_exists
3 not_authorized
4 username_required
5 bad_method
6 password_required
7 usergroup_dont_exist
8 bad_time_rage
9 bad_usertype
10 update_error
99 others
These exceptions are explained in the documentation on the functionmodules SUSR_RFC_USER_INTERFACE.

System Response
The function terminates.

Procedure
Remove the cause of the error and execute the command again.