I have found that it is sometimes(though not always) necessary to make a call to AddConnection to add a network connection to the UNC before I can open the file using a UNC. You can use AddConnection to add a connection without mapping a drive by not supplying a local device name. This is the work around that I have settled on and it does work although it requires a few more lines of code and final call to CancelConnection.

I have never put the time into figuring out why AddConnection is sometimes required: I have just found from experience that it sometimes is - even when using domain accounts or local accounts on two machines that use the same login credentials and identical access rights on the target directory and file.

I would be interested in the answer to this if anybody does know.