We are experiencing this problem on a server that was upgraded from version 4 to version 5. When exporting the document library (LAR) it looks like that if a file is missing the whole process stops.
We are not experiencing this issue on a fresh 5.2.3 installation .
I have done some investigation : in com.liferay.documentlibrary.util.JCRHook the call
contentNode = fileNode.getNode(JCRConstants.JCR_CONTENT);
throws a PathNotFoundException which propagates up to a com.liferay.documentlibrary.NoSuchFileException: blocking the export completely.
We are in need of exporting the data to import it in a new server ( at this stage we are willing to import the data in a fresh liferay installation ) and we are talking about hundreds of documents : this behaviour is not exaclty what we expected.