I've tried to create a serial letter with the "Mail Merge Wizard" (Tools -> Mail Merge Wizard ...) but without any success. Firstly I assumed, that this is a bug in the app itself. But this assumption wasn't true, because I can create a serial letter with the portable version X-LibreOffice.
The failure occurs after step 6 "Edit document" within the "Mail Merge Wizard". Now the inserted address block (see step 3 of the "Mail Merge Wizard") disappears.