I've had a VFP6 app exchanging data with QuickBooks using QBFC for about 11 years and recently updated it to VFP9.
Now, the app cannot connect to QuickBooks even though there has been no change in the code.
QBSessionManager.OpenConnection() now returns .NULL. with no error message at all and no response from QuickBooks.

Using the same QB company file on the same machine with the same version of QBFC, the VFP6 version connects and the VFP9 version does not.
Does anyone have any idea what I can look for on this?

Wish I could be more helpful, but I can tell you that this code works just fine in VFP 9 for me:

loSession = CREATEOBJECT("QBFC10.QBSessionManager")
*** Get the name of the app from the ini file
lcAppName = oIniMgr.GetValue( [QUICKBOOKS], [AppName], [appUser.ini] )
loSession.OpenConnection("", lcAppName )

You may also find it helpful to download the white paper and code samples from my conference session QuickBooks Automation