thanks for your reply, i had a closer look now and found, that my default.conf had some suspicious values:
<main_frame>
<layout>
<TOOLBAR_SHOW bool="1" />
<LEFT_BLOCK_SELECTION int="2" />
<BOTTOM_BLOCK_SELECTION int="4" />
<MAXIMIZED bool="0" />
<LEFT int="1353" />
<TOP int="66" />
<WIDTH int="1144" />
<HEIGHT int="958" />
</layout>
i deleted this section and the debug-toolbar came back
[edit]
most probably the problem was located in the
<LAYOUT>
<bin crc= ... </bin>
</LAYOUT>
section, which i also deleted.