Version: Unbekannt (using KDE 3.5.5, Kubuntu (edgy) 4:3.5.5-0ubuntu3.1)
Compiler: Target: i486-linux-gnu
OS: Linux (i686) release 2.6.17-11-generic
Bug 90518 describes the same problem. Deactivating the anti spam filters ("mark this as spam"/"mark thsi as ham") in the toolbar and the manual filters menu is a workaround but it is still far from being fixed. I am using kontact 1.2.3 from the most recent kubuntu update in KDE version 3.5.5. Using the Anti-Spam Assistant to set up the filters reproduces the bug.
Do you have a backtrace? (install kdelibs-dbg and kdepim-dbg and try to reproduce and paste the backtrace here)
Never had this bug again at least since moving to feisty.