Workaround found:

i have to put a non-empty element before my first comment and the remaining content survives! - comments before the letter B in Bannerdummy of course are stripped :-)!!

the second div in
<div style="display: none">Bannerdummy</div>
<div><!--banner starts--><em>Hello World</em><!--banner ends--></div>

is not touched, as long the first div is not empty :-)

please, fix this one, this workaround works for now (for today), but this is of course crap!