SQL database, C/Front and Windows Authentication

I have a client who want to use Jetreports to access a SQL database with Windows Authentication. The issue was that after Jetreports was configured with C/Front, 5 users could not connect to the SQL database and got a C/Front error saying the users did not have permission to access the database. All 5 users were designated superuser status. The Navision client works fine with Windows authentication for these users and when assigned Database authentication, they work fine too. Jetreports suggested deleting the SQL user login and Navision login and recreating them which did the trick - all 5 users were able to connect with Windows authentication. Here’s the problem: 1 month later, they have now notified me that the problem is back for 3 users. 1 user was from the original group of 5 and 2 users (also superusers) were not. The client is now suggesting it is Jetreports or C/Front that is the issue and are asking for an explanation of why this is happening. They want to stay on windows authentication and not switch to database authentication. Question is since deleting the SQL and Navision Logins and recreating them seemed to work initially, would this be a recurring SQL corruption issue? If so, why would the Navision client work? Are there any tools I can use to get more detail into why this is happening, as the C/Front permissions error is not very descriptive. Any advice would be greatly appreciated. Thanks.