Thanks for the bug report! I could not reproduce this in trunk (maybe because there's nothing in my "Network" folder). Can you reproduce this every time you try? And if yes, is your view split when you middle-click "Network"? I'm asking because the assert that you got should only be hit in a split view setup as far as I know.

When clinking on "network" (not middle click) it opens correctly. When middle-clicking dolphin still crashs.
When running dolphin from root user (kdesu dolphin) the problem doesn't appear.

Here is the last trace:

Application: Dolphin (dolphin), signal: Aborted
[KCrash Handler]
#6  0xb80d3430 in __kernel_vsyscall ()
#7  0xb66ca6d0 in raise () from /lib/tls/i686/cmov/libc.so.6
#8  0xb66cc098 in abort () from /lib/tls/i686/cmov/libc.so.6
#9  0xb6998f9d in qt_message_output () from /opt/project-neon/lib/libQtCore.so.4
#10 0xb6999057 in qFatal () from /opt/project-neon/lib/libQtCore.so.4
#11 0xb69993f5 in qt_assert () from /opt/project-neon/lib/libQtCore.so.4
#12 0x080713e7 in DolphinViewContainer::isActive (this=0x98e7ce8) at /build/buildd/kde-nightly-kdebase-20090623+svn985907/apps/dolphin/src/dolphinviewcontainer.cpp:219
#13 0x080719b3 in DolphinViewContainer::slotDirListerCompleted (this=0x3) at /build/buildd/kde-nightly-kdebase-20090623+svn985907/apps/dolphin/src/dolphinviewcontainer.cpp:291
#14 0x08071dfa in DolphinViewContainer::qt_metacall (this=0x98e7ce8, _c=QMetaObject::InvokeMetaMethod, _id=4, _a=0xbfeef9c8)
    at /build/buildd/kde-nightly-kdebase-20090623+svn985907/obj-i486-linux-gnu/apps/dolphin/src/dolphinviewcontainer.moc:115
#15 0xb6aa90e8 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#16 0xb6aa9722 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#17 0xb7f57917 in KDirLister::completed (this=0x98e4af8) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/kdirlister.moc:207
#18 0xb7f5e532 in KDirListerCache::slotResult (this=0x9595d78, j=0x987a660) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/kdirlister.cpp:1154
#19 0xb7f661ac in KDirListerCache::qt_metacall (this=0x9595d78, _c=QMetaObject::InvokeMetaMethod, _id=8, _a=0xbfeefbcc)
    at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/kdirlister_p.moc:99
#20 0xb6aa90e8 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#21 0xb6aa9722 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#22 0xb750eaa3 in KJob::result (this=0x987a660, _t1=0x987a660) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kdecore/kjob.moc:188
#23 0xb750efde in KJob::emitResult (this=0x987a660) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kdecore/jobs/kjob.cpp:304
#24 0xb7f3a218 in KIO::SimpleJob::slotFinished (this=0x987a660) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/job.cpp:477
#25 0xb7f3a37c in KIO::ListJob::slotFinished (this=0x987a660) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/job.cpp:2408
#26 0xb7f3f3eb in KIO::ListJob::qt_metacall (this=0x987a660, _c=QMetaObject::InvokeMetaMethod, _id=3, _a=0xbfeefdd8)
    at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/jobclasses.moc:764
#27 0xb6aa90e8 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#28 0xb6aa9722 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#29 0xb7feea77 in KIO::SlaveInterface::finished (this=0x96c64c0) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/slaveinterface.moc:165
#30 0xb7ff0d17 in KIO::SlaveInterface::dispatch (this=0x96c64c0, _cmd=104, rawdata=@0xbfeeff74) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/slaveinterface.cpp:175
#31 0xb7ff114d in KIO::SlaveInterface::dispatch (this=0x96c64c0) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/slaveinterface.cpp:91
#32 0xb7fe4797 in KIO::Slave::gotInput (this=0x96c64c0) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/slave.cpp:322
#33 0xb7fe5b93 in KIO::Slave::qt_metacall (this=0x96c64c0, _c=QMetaObject::InvokeMetaMethod, _id=2, _a=0xbfef0088)
    at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/slave.moc:76
#34 0xb6aa90e8 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#35 0xb6aa9722 in QMetaObject::activate () from /opt/project-neon/lib/libQtCore.so.4
#36 0xb7f0b707 in KIO::Connection::readyRead (this=0x94db218) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/connection.moc:86
#37 0xb7f0c5e6 in KIO::ConnectionPrivate::dequeue (this=0x94db218) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kio/kio/connection.cpp:82
#38 0xb7f0d436 in KIO::Connection::qt_metacall (this=0x94db218, _c=QMetaObject::InvokeMetaMethod, _id=1, _a=0x9935b88)
    at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/obj-i486-linux-gnu/kio/connection.moc:73
#39 0xb6aa1a2b in QMetaCallEvent::placeMetaCall () from /opt/project-neon/lib/libQtCore.so.4
#40 0xb6aa3c90 in QObject::event () from /opt/project-neon/lib/libQtCore.so.4
#41 0xb6c727fc in QApplicationPrivate::notify_helper () from /opt/project-neon/lib/libQtGui.so.4
#42 0xb6c7b44f in QApplication::notify () from /opt/project-neon/lib/libQtGui.so.4
#43 0xb783d423 in KApplication::notify (this=0xbfef07fc, receiver=0x94db218, event=0x99333e0) at /build/buildd/kde-nightly-kdelibs-20090623+svn985907/kdeui/kernel/kapplication.cpp:302
#44 0xb6a92b3b in QCoreApplication::notifyInternal () from /opt/project-neon/lib/libQtCore.so.4
#45 0xb6a964e5 in QCoreApplicationPrivate::sendPostedEvents () from /opt/project-neon/lib/libQtCore.so.4
#46 0xb6a966ed in QCoreApplication::sendPostedEvents () from /opt/project-neon/lib/libQtCore.so.4
#47 0xb6abdabf in ?? () from /opt/project-neon/lib/libQtCore.so.4
#48 0xb62f0b88 in g_main_context_dispatch () from /usr/lib/libglib-2.0.so.0
#49 0xb62f40eb in ?? () from /usr/lib/libglib-2.0.so.0
#50 0xb62f4268 in g_main_context_iteration () from /usr/lib/libglib-2.0.so.0
#51 0xb6abd7a8 in QEventDispatcherGlib::processEvents () from /opt/project-neon/lib/libQtCore.so.4
#52 0xb6d0d065 in ?? () from /opt/project-neon/lib/libQtGui.so.4
#53 0xb6a9157a in QEventLoop::processEvents () from /opt/project-neon/lib/libQtCore.so.4
#54 0xb6a9173a in QEventLoop::exec () from /opt/project-neon/lib/libQtCore.so.4
#55 0xb6a967b1 in QCoreApplication::exec () from /opt/project-neon/lib/libQtCore.so.4
#56 0xb6c72527 in QApplication::exec () from /opt/project-neon/lib/libQtGui.so.4
#57 0x08078fc9 in main (argc=6, argv=0xbfef09b4) at /build/buildd/kde-nightly-kdebase-20090623+svn985907/apps/dolphin/src/main.cpp:94

>>> Can you reproduce this every time you try?
>>> And if yes, is your view split when you middle-click "Network"?
>>> I'm asking because the assert that you got should only be hit
>>> in a split view setup as far as I know.

Yes, it happens every time I try.
No, I don't have the view splitted.

- Is this still happening on KDE4.3 or an updated 4.4 build ?
Thanks

*** Bug 214890 has been marked as a duplicate of this bug. ***

Maybe this is related to bug 213895 - all frames starting from #18 also appear in the backtrace of that bug.

I can not reproduce this bug anymore.
Should I close it?

Yes..
Closing