Titles for Blog posts not showing: where to look?

On my website http://www.gloeiendekolen.nl, using the Free Nictitate Theme, for some reason the post titles won’t show. All posts use a featured image and I have already tried disabling all plugings. But for the life of me, I can’t find what is causing this behaviour. From what I can gather from the page source, the title is generated but it does not want to show itself.

This has been bugging me for some time now and has cost me at least a day troubleshooting, but so far I have not found a solution. Does anyone have any idea what could cause this behaviour and where to look for and correct the code?

Thanks,

Mark