The reason for the 'File Access Denied' is that Adagio Invoices is looking for OE (to determine if the OE printer should be loaded). Because in OE, the Order screen is open, some resources are already in use, and Adagio Invoices isn't too happy about that. If the OE order screen wasn't open, Invoices would open OK.

There probably should be a better way for Invoices to determine if OE is open, so this is something we'll try and look at for a future release. In the meantime, is there a reason why you need the OE order window open when printing invoices/picking slips?