Version:            (using KDE KDE 3.5.5)
Installed from:    Gentoo Packages
Compiler:          gcc (GCC) 4.1.1 (Gentoo 4.1.1-r1) 
OS:                Linux

If I try to add the Amarok sidebar to the Navigation Panel in Konqueror, Konqueror freezes.

It keeps printing error messages:
...
377312
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377313
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377314
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377315
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377316
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377317
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377318
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377319
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377320
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377321
QString::arg(): Argument missing: /konqsidebartng/webbrowsing/entries/amarok.desktop, 377322

If I attach GDB and look at the backtrace, I sometimes get something like this:
#0  0xb6a98b00 in operator delete () from /usr/lib/gcc/i686-pc-linux-gnu/4.1.1/libstdc++.so.6
#1  0xb7321f54 in QStringData::deleteSelf () from /usr/qt/3/lib/libqt-mt.so.3
#2  0xb76c3506 in KStandardDirs::saveLocation () from /usr/kde/3.5/lib/libkdecore.so.4
#3  0xb76c4ee1 in locateLocal () from /usr/kde/3.5/lib/libkdecore.so.4
#4  0xb76c4f9f in locateLocal () from /usr/kde/3.5/lib/libkdecore.so.4
#5  0xb5ee0734 in findFileName (tmpl=<value optimized out>, universal=<value optimized out>, profile=@0x85a9e28)
    at sidebar_widget.cpp:155
#6  0xb5ee3cfd in addBackEnd::activatedAddMenu (this=0x85a9dd0, id=5) at sidebar_widget.cpp:196
#7  0xb5ee411e in addBackEnd::qt_invoke (this=0x85a9dd0, _id=3, _o=0xbfa29d94) at sidebar_widget.moc:181
#8  0xb7058f20 in QObject::activate_signal () from /usr/qt/3/lib/libqt-mt.so.3
#9  0xb7059b12 in QObject::activate_signal () from /usr/qt/3/lib/libqt-mt.so.3
#10 0xb73a1720 in QPopupMenu::activated () from /usr/qt/3/lib/libqt-mt.so.3
#11 0xb715b932 in QPopupMenu::actSig () from /usr/qt/3/lib/libqt-mt.so.3
#12 0xb715becb in QPopupMenu::mouseReleaseEvent () from /usr/qt/3/lib/libqt-mt.so.3
#13 0xb70912ea in QWidget::event () from /usr/qt/3/lib/libqt-mt.so.3
#14 0xb6ffa447 in QApplication::internalNotify () from /usr/qt/3/lib/libqt-mt.so.3
#15 0xb6ffb199 in QApplication::notify () from /usr/qt/3/lib/libqt-mt.so.3
#16 0xb7715bee in KApplication::notify () from /usr/kde/3.5/lib/libkdecore.so.4
#17 0xb6f9b9d8 in QETWidget::translateMouseEvent () from /usr/qt/3/lib/libqt-mt.so.3
#18 0xb6f9a8ef in QApplication::x11ProcessEvent () from /usr/qt/3/lib/libqt-mt.so.3
#19 0xb6faa93a in QEventLoop::processEvents () from /usr/qt/3/lib/libqt-mt.so.3
#20 0xb7010b40 in QEventLoop::enterLoop () from /usr/qt/3/lib/libqt-mt.so.3
#21 0xb6ff9e4f in QApplication::enter_loop () from /usr/qt/3/lib/libqt-mt.so.3
#22 0xb715f0c7 in QPopupMenu::exec () from /usr/qt/3/lib/libqt-mt.so.3
#23 0xb5edd997 in Sidebar_Widget::mousePressEvent (this=0x8330f78, ev=0x8903358) at sidebar_widget.cpp:863
#24 0xb7091310 in QWidget::event () from /usr/qt/3/lib/libqt-mt.so.3
#25 0xb6ffa447 in QApplication::internalNotify () from /usr/qt/3/lib/libqt-mt.so.3
#26 0xb6ffb199 in QApplication::notify () from /usr/qt/3/lib/libqt-mt.so.3
#27 0xb7715bee in KApplication::notify () from /usr/kde/3.5/lib/libkdecore.so.4
#28 0xb6f9b022 in QETWidget::translateMouseEvent () from /usr/qt/3/lib/libqt-mt.so.3
#29 0xb6f9a8ef in QApplication::x11ProcessEvent () from /usr/qt/3/lib/libqt-mt.so.3
#30 0xb6faa93a in QEventLoop::processEvents () from /usr/qt/3/lib/libqt-mt.so.3
#31 0xb7010b40 in QEventLoop::enterLoop () from /usr/qt/3/lib/libqt-mt.so.3
#32 0xb70109c6 in QEventLoop::exec () from /usr/qt/3/lib/libqt-mt.so.3
#33 0xb6ff9edf in QApplication::exec () from /usr/qt/3/lib/libqt-mt.so.3
#34 0xb7f0903b in kdemain (argc=1, argv=0xbfa2b3e4) at konq_main.cc:206
#35 0x08048722 in main (argc=Cannot access memory at address 0x1
) at konqueror.la.cc:2

But most of the time it's this:
#0  0xb7f46410 in __kernel_vsyscall ()
#1  0xb6934793 in write () from /lib/libc.so.6
#2  0xb68d2871 in _IO_file_write () from /lib/libc.so.6
#3  0xb68d2537 in _IO_file_seek () from /lib/libc.so.6
#4  0xb68d27e9 in _IO_file_xsputn () from /lib/libc.so.6
#5  0xb68b1552 in vfprintf () from /lib/libc.so.6
#6  0xbfa27828 in ?? ()
#7  0xb6bf6b24 in ?? () from /usr/lib/libX11.so.6
#8  0x00000000 in ?? ()