Hi,
what's NAV release ?
in your report two dataitems inserted: the first dataitem linked to lines and the second linked to header ? correct ?
May be a problem with too many lines with different date values in first dataitem, you need skip data in this case and not using showoutput false; verify also links and filter applied with debug session.
If you want send me the report (fob object), i can make some test for you.