[TYPO3] How do I disable the RTE for the HTML content type?

[tapio]

> I'm running on PHP 4.x.  Noting special there.

Ok. good to know that it is not related with that

> I have seen a thread on this forum (or the dev forum) some time ago 
> about this but I can't find it any more. 
>  I thought it had something to
> do with an extension messing things up.

Well I asked this issues once also in this forum - and I got answer that
some extension messing things up - but no idea what extension.