If users do not have Edit My Own Posts enabled on their profile, they will have issues saving a document to Chatter (&SC1=Chatter) when running Conga Composer Release 8 in background mode. A Chatter post will be logged, but there will be an error message instead of a document that says:
'Error creating FeedAttachment 0<br />Error code is: INSUFFICIENT_ACCESS_ON_CROSS_R
What to Do
To correct this, enable Edit My Own Posts in System Wide Permissions (see below) for all affected user profiles.