Oke, after many hours debugging and doing tons of research and Google queries. I finaly solved my problem (for now and a bit dirty). It appeared to be a bug in the Zend Markup library cause of the fact that it was some last quick and rushed code right before one of the releases of ZF1. http://www.dasprids.de/blog/2010/02/28/why-i-am-not-using-zend_markup
Solution: Comment out rule 312 in the Zend_Markup_Renderer_RendererAbstract class.
I will move in a little deeper into this subject when this project is finished. A quite a relief is that the Zend_Markup bug is or will get fixed in ZF2.