after changing my content type to formated text my broswer cant access the content page anymore (http 500 error)

I will try to make it clear, my installation is in french.

Ok I have a content type and when I go in the Fields and change in the configuration of that field to Filtered text (User choose the format) and then try accessing a content using that content type I dont have access to that content, its take a long time to load and finally I got an http 500 error (Remote server or file not found)

whats worse is when I try to rechange that field in my content type here too I dont have access anymore... the only I found was to make back on my browser (or use a new browser) to change it to Simple text... after doing that everything is right back to normal

any idea? I need that field to have HTML in it

thanks