[TYPO3] Section index stopped working

[Peter Huppertz]

I'm using the content type "setion index" to display an overview of the 
contents of some pages. This went well for a long time, but now it stops 
working. The content element is rendered (I can see the headline and the 
html comments in the source code) but doesn't show the various headings. 
All headings have "index" enabled and the anchors are sucessfully rendered.
I'm working with Typo3 4.0.1 and TV 1.1.0. I tried to disable all 
extensions I installed recently but that doesn't solve the problem.
Any suggestions why this happens or any possibilities to debug this problem?

Thanks,

Peter