If you are having trouble printing mail merge or labels from LibreOffice (see this bug report here), there is a workaround:
1. Set up your merged document.
2. Save the set up document without trying to merge it
3. Close all LibreOffice applications. Kill the daemon if you have it running (I don’t).
4. Re-open your set-up document. It will ask you if you wish to reconnect to sources. Answer yes.