The login failed error message that SQL Server returns to the client is vague by design (to prevent hackers exploiting the information they are given).

You might find more information on the exact reason of the login failure by checking the errorlog around the time of the error. There might be more details recorded there.

If that does not help you, then you will have to set up a trace (extended events session if you're on SQL2012 or higher) to see what is going on when you start that ERP module.

With this type of problem, the solution is usually obvious once you know the cause.

Also, not really an answer to your question and perhaps something you cannot influence because it's baked into your ERP package but I am going to say it anyway: connecting to SQL Server on the sa user is a security risk and hence not recommend.