launcher.c:launcher_update_panel_entry on the 4.4 branch defines char tip[512], but the subsequent g_snprintf and g_strlcpy use 521 as the length. This ends up corrupting the stack (at least on HPUX) which causes the panel to crash. Updating the values to 512 fixes the issue.
Better use sizeof(tip) in the g_snprintf and g_strlcpy functions.
Seems I was temporarily dyslectic there... I committed a fix with SVN revision 25792, so the problem should be solved in the next release of Xfce. Thanks a lot for the report.