hmm It's hard to say without more information or code samples... I'm not familiar with that error but I guess there could be 100 reasons why a document might not be opened...
This won't help you too quickly but In august I'm going to do a session at MWLug on working with Attachments in XPages. I put the abstract below. That content will certainly be coming to NotesIn9 in September and possibly even to OpenNTF. So that might be helpful to you at that point.
Enhance your XPages app with Files, Images and More
Let’s say you want to build a website for a bicycle store. You have specifications for the bikes but what about supporting files? Each bike needs it’s own pictures, manuals and parts lists. Maybe reviews and warranty forms as well. How do you keep everything linked together and still be easily usable and scalable? Come to this session to learn how to add a multiple file uploader AND an image gallery to your page. See how you can use Java to not only move files to specific databases but also to resize your pictures to common sizes.