Thanks for your Tipps!
Since it is repeatable in your machine, you can compile a new postgres version with "--enable-cassert" (enable assertions in code) and "--enable-debug" (enable gcc debug support) configuration. Then run it on your data and "bt" the core dump.
I try to found out the reason for that behavoir. For now i could drop this damaged table und restore it from an older backup, so all works fine again. regards, thomas!
