If the problem is with an additional admin account that you created after setup was complete, the issue could be that the account does not have an assigned role. Provided that the super admin account that you created during the setup wizard still works, login and go to:
Administration
Admins and Roles
If needed, click the 2nd tab "Administrators" and create the new account (name, password, etc)
Next click the 1st tab "Create User Group"
Give the group a name (eg SZ Admins)
Assign the permission level (AP Admin, GuestPass Admin, Network Admin, Super Admin, etc...)
Assign the Resources that the group will have access to (SZ, APs, other admin accounts, etc)
Assign the new account that you created earlier to the group
The above is for newer versions (maybe 3.4 and later I think...). If you're running an older version of SZ code (maybe 3.2 or earlier), the process is a little different.