the preview rarely works right.
for a simple test I just made, the preview showed wrong with HTML but right with UBB+HTML.

I wouldn't worry about the postprep anyways, as:
a) the board is better with code tags nowadays
b) the udf-site http://www.kixtart.org/udf works perfect, even if the board fails
c) korg has coloring with the code-tags enhancement, if you want colors. inside code-tags insert [postprep] tag as well, and your code will be pimped out
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