Well, in a document with about 16 chapters and lots of subchapters I
use graphs. Below these Graphs are captions. Everything worked fine
until now. Suddenly, all captions start with a numbering of
0.something, e.g. 0.35 for the 35th picture, whereas before they
included the correct chapter number.

If I create a new caption of a new picture and click the "include
chapter number" option for the caption, a message pops up:

"There is no chapter number to include in the caption or page
number..."

How is this possible, I have a lot of chapter numbers based of the
built-in styles?