This is what I get from gdb:

#0  0xb7f63410 in __kernel_vsyscall ()
#1  0xb6601f23 in __write_nocancel () from /lib/libc.so.6
#2  0xb65b4377 in _IO_new_file_write () from /lib/libc.so.6
#3  0xb65b4055 in new_do_write () from /lib/libc.so.6
#4  0xb65b42e9 in _IO_new_file_xsputn () from /lib/libc.so.6
#5  0xb65948b4 in buffered_vfprintf () from /lib/libc.so.6
#6  0xb659077f in vfprintf () from /lib/libc.so.6
#7  0xb65992f2 in fprintf () from /lib/libc.so.6
#8  0xb7148292 in qWarning (msg=0xb7258950 "QString::arg(): Argument missing: %s, %llu") at qglobal.cpp:545
#9  0xb716ae48 in QString::arg (this=0xbfafb2fc, a=0x172b, fieldWidth=0x0, base=0xa) at qstring.cpp:2134
#10 0xb716aff4 in QString::arg (this=0xbfafb2fc, a=0x172b, fieldWidth=0x0, base=0xa) at qstring.cpp:2088
#11 0xb5d88d2b in findFileName () from /usr/kde/3.5/lib/kde3/konq_sidebar.so
#12 0xb5d8b36d in addBackEnd::activatedAddMenu () from /usr/kde/3.5/lib/kde3/konq_sidebar.so
#13 0xb5d8b9c2 in addBackEnd::qt_invoke () from /usr/kde/3.5/lib/kde3/konq_sidebar.so
#14 0xb6de507c in QObject::activate_signal (this=0x8486ac8, clist=0x8487bc8, o=0xbfafb588) at qobject.cpp:2380
#15 0xb6de5ace in QObject::activate_signal (this=0x8486ac8, signal=0x2, param=0x1) at qobject.cpp:2449
#16 0xb71ff5a7 in QPopupMenu::activated (this=0x8486ac8, t0=0x1) at moc_qpopupmenu.cpp:151
#17 0xb6f2c967 in QPopupMenu::actSig (this=0x8486ac8, id=0x1, inwhatsthis=0x0) at qpopupmenu.cpp:724
#18 0xb6f2cfeb in QPopupMenu::mouseReleaseEvent (this=0x8486ac8, e=0xbfafbb14) at qpopupmenu.cpp:1689
#19 0xb6e25c13 in QWidget::event (this=0x8486ac8, e=0xbfafbb14) at qwidget.cpp:4677
#20 0xb6d7075b in QApplication::internalNotify (this=0xbfafcdbc, receiver=0x8486ac8, e=0xbfafbb14) at qapplication.cpp:2635
#21 0xb6d72a59 in QApplication::notify (this=0xbfafcdbc, receiver=0x8486ac8, e=0xbfafbb14) at qapplication.cpp:2421
#22 0xb75b0dee in KApplication::notify () from /usr/kde/3.5/lib/libkdecore.so.4
#23 0xb6cf97bb in QApplication::sendSpontaneousEvent (receiver=0x8486ac8, event=0xbfafbb14) at qapplication.h:499
#24 0xb6cf8111 in QETWidget::translateMouseEvent (this=0x8486ac8, event=0xbfafbfb8) at qapplication_x11.cpp:4238
#25 0xb6cf65b2 in QApplication::x11ProcessEvent (this=0xbfafcdbc, event=0xbfafbfb8) at qapplication_x11.cpp:3447
#26 0xb6d0f032 in QEventLoop::processEvents (this=0x80c2a98, flags=0x4) at qeventloop_x11.cpp:192
#27 0xb6d8e775 in QEventLoop::enterLoop (this=0x80c2a98) at qeventloop.cpp:198
#28 0xb6d7239d in QApplication::enter_loop (this=0xbfafcdbc) at qapplication.cpp:2793
#29 0xb6f2df80 in QPopupMenu::exec (this=0x84c8db0, pos=@0xbfafc210, indexAtPoint=0x0) at qpopupmenu.cpp:2424
#30 0xb5d85bbb in Sidebar_Widget::eventFilter () from /usr/kde/3.5/lib/kde3/konq_sidebar.so
#31 0xb6de43a7 in QObject::activate_filters (this=0x84a26e8, e=0xbfafc6c4) at qobject.cpp:903
#32 0xb6de4448 in QObject::event (this=0x84a26e8, e=0xbfafc6c4) at qobject.cpp:735
#33 0xb6e25b2a in QWidget::event (this=0x84a26e8, e=0xbfafc6c4) at qwidget.cpp:4659
#34 0xb6d7075b in QApplication::internalNotify (this=0xbfafcdbc, receiver=0x84a26e8, e=0xbfafc6c4) at qapplication.cpp:2635
#35 0xb6d72a59 in QApplication::notify (this=0xbfafcdbc, receiver=0x84a26e8, e=0xbfafc6c4) at qapplication.cpp:2421
#36 0xb75b0dee in KApplication::notify () from /usr/kde/3.5/lib/libkdecore.so.4
#37 0xb6cf97bb in QApplication::sendSpontaneousEvent (receiver=0x84a26e8, event=0xbfafc6c4) at qapplication.h:499
#38 0xb6cf841c in QETWidget::translateMouseEvent (this=0x84a26e8, event=0xbfafcb68) at qapplication_x11.cpp:4296
#39 0xb6cf65b2 in QApplication::x11ProcessEvent (this=0xbfafcdbc, event=0xbfafcb68) at qapplication_x11.cpp:3447
#40 0xb6d0f032 in QEventLoop::processEvents (this=0x80c2a98, flags=0x4) at qeventloop_x11.cpp:192
#41 0xb6d8e775 in QEventLoop::enterLoop (this=0x80c2a98) at qeventloop.cpp:198
#42 0xb6d8e596 in QEventLoop::exec (this=0x80c2a98) at qeventloop.cpp:145
#43 0xb6d7241f in QApplication::exec (this=0xbfafcdbc) at qapplication.cpp:2758
#44 0xb7ecee19 in kdemain () from /usr/kde/3.5/lib/libkdeinit_konqueror.so
#45 0x08048722 in main ()
#46 0x00000001 in ?? ()
#47 0xbfafd264 in ?? ()
#48 0x00000000 in ?? ()

It was looping outputting these lines:
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 5883
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 5884
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 5885
...

*** Bug 137946 has been marked as a duplicate of this bug. ***

Is this bug still valid for KDE?

Dear Bug Submitter,

This bug has been in NEEDSINFO status with no change for at least 15 days. Please provide the requested information as soon as possible and set the bug status as REPORTED. Due to regular bug tracker maintenance, if the bug is still in NEEDSINFO status with no change in 30 days, the bug will be closed as RESOLVED > WORKSFORME due to lack of needed information.

For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

If you have already provided the requested information, please set the bug status as REPORTED so that the KDE team knows that the bug is ready to be confirmed.

Thank you for helping us make KDE software even better for everyone!

This bug has been in NEEDSINFO status with no change for at least
30 days. The bug is now closed as RESOLVED > WORKSFORME
due to lack of needed information.

For more information about our bug triaging procedures please read the
wiki located here:
https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging

Thank you for helping us make KDE software even better for everyone!