OK I have a lead now. I have turned off all the plugins apart from CB core and the templates but I still have the error and the rendering of more than 15 seconds.
Here is what I have found though:
I had saved 3 photos in CB gallery. These photos where now throwing errors (since I switched off gallery, btw no idea why they were loaded in the edit settings). The names have attached an infinity of orig. endings, which I guess is overloading php (PCRE.Backtrack_limit exceeded) and are slowing down the website massively (see attached screenshot).
Furthermore, the website was throwing an "Unexpected synthax error" in Programm 4, which disappeared when I disabled CB connections or Groupjive. It is not the error I was looking for but thought it would be relevant as well (I also have this error in my saigon website which is working fine).