Hi,
I`ve downloaded the Debian package for KTorrent 1.2 and installed it over the currenly installed 1.1

I got a dialog saying that my torrents in progress will be updated, and after that I got a SIGABRT.

Here is the console output:
ilia@lan:~$ ktorrent
terminate called after throwing an instance of 'bt::Error'
ERROR: Communication problem with ktorrent, it probably crashed.
ilia@lan:~$ KCrash: Application 'ktorrent' crashing...

Here is the GDB output:
(no debugging symbols found)
Using host libthread_db library "/lib/tls/libthread_db.so.1".
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
(no debugging symbols found)
[Thread debugging using libthread_db enabled]
[New Thread -1233541440 (LWP 17124)]
[KCrash handler]
#5 0xb67e67c7 in raise () from /lib/tls/libc.so.6
#6 0xb67e806b in abort () from /lib/tls/libc.so.6
#7 0xb69d8de9 in __gnu_cxx::__verbose_terminate_handler ()
from /usr/lib/libstdc++.so.6
#8 0xb69d6765 in __gxx_personality_v0 () from /usr/lib/libstdc++.so.6
#9 0xb69d67a2 in std::terminate () from /usr/lib/libstdc++.so.6
#10 0xb69d696f in __cxa_rethrow () from /usr/lib/libstdc++.so.6
#11 0xb7e9f587 in bt::MultiFileCache::downloadStatusChanged ()
from /usr/lib/libktorrent.so.0
#12 0xb7e74cab in bt::ChunkManager::downloadStatusChanged ()
from /usr/lib/libktorrent.so.0
#13 0xb7e74f14 in bt::ChunkManager::qt_invoke ()
from /usr/lib/libktorrent.so.0
#14 0xb6fbeb57 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#15 0xb7ea552b in bt::TorrentFile::downloadStatusChanged ()
from /usr/lib/libktorrent.so.0
#16 0xb7ea5637 in bt::TorrentFile::setDoNotDownload ()
from /usr/lib/libktorrent.so.0
#17 0xb7e7542d in bt::ChunkManager::loadFileInfo ()
from /usr/lib/libktorrent.so.0
#18 0xb7e754b2 in bt::ChunkManager::loadIndexFile ()
from /usr/lib/libktorrent.so.0
#19 0xb7e886ba in bt::TorrentControl::init () from /usr/lib/libktorrent.so.0
#20 0x0806b0e7 in kt::CompareVal<unsigned long> ()
#21 0x0806b539 in kt::CompareVal<unsigned long> ()
#22 0x0805d991 in QGList::count ()
#23 0x0806f743 in QValueList<QString>::detachInternal ()
#24 0xb76f6df8 in KUniqueApplication::processDelayed ()
from /usr/lib/libkdecore.so.4
#25 0xb76f869c in KUniqueApplication::qt_invoke ()
from /usr/lib/libkdecore.so.4
#26 0x0806f37f in QValueList<QString>::detachInternal ()
#27 0xb6fbeb57 in QObject::activate_signal () from /usr/lib/libqt-mt.so.3
#28 0xb734f055 in QSignal::signal () from /usr/lib/libqt-mt.so.3
#29 0xb6fdca40 in QSignal::activate () from /usr/lib/libqt-mt.so.3
#30 0xb6fe4354 in QSingleShotTimer::event () from /usr/lib/libqt-mt.so.3
#31 0xb6f54698 in QApplication::internalNotify () from /usr/lib/libqt-mt.so.3
#32 0xb6f548b6 in QApplication::notify () from /usr/lib/libqt-mt.so.3
#33 0xb76e4d4e in KApplication::notify () from /usr/lib/libkdecore.so.4
#34 0xb6ee45e5 in QApplication::sendEvent () from /usr/lib/libqt-mt.so.3
#35 0xb6f4598c in QEventLoop::activateTimers () from /usr/lib/libqt-mt.so.3
#36 0xb6ef835c in QEventLoop::processEvents () from /usr/lib/libqt-mt.so.3
#37 0xb6f6cda2 in QEventLoop::enterLoop () from /usr/lib/libqt-mt.so.3
#38 0xb6f6cccb in QEventLoop::exec () from /usr/lib/libqt-mt.so.3
#39 0xb6f53225 in QApplication::exec () from /usr/lib/libqt-mt.so.3
#40 0x0805b7e6 in ?? ()
#41 0xb67d1eb0 in __libc_start_main () from /lib/tls/libc.so.6
#42 0x0805b491 in ?? ()

The distribution is Debian-based Tilix 1.0.1 - with dist-upgrade to latest sid.

That should be fixed in current SVN.