From what you describe, it sounds like the user receiving the error may not have sufficient rights to the Titanium database. First thing to check would be the SQL Logins. If you have an AD group mapped to a SQL login for the Titanium database, make sure that the user's AD account is added to that group. Additionally, make sure that the SQL login has the correct database roles. The only roles that should be checked are 'public' and 'TiConnect'. Please see the image below:
If my suggestions do not resolve the issue, please send a screenshot of the errors the user is receiving when trying to reach the Titanium Schedule login screen.