Shrug. This looks like a KOMA bug to me. If you nest the dictum environment into a chapterpreamble, it works. You have to use some ERT like in the attached file and place the whole thing *above* the chapter heading.
BTW: You can toggle dictum fonts with \setkomafont{dictumtext}{\normalfont\normalcolor\small} in the preamble.
write into the preamble
\let\dictumOld\dictum
\renewcommand\dictum[1]{{\dictumOld{#1}}}Herbert
