Thanks for your answer. I used to use your suggestion with rtmpdump before, but now the page in question does no longer serve a mp4-file and does not employ the rtmp protocol.

Instead, they changed to 'embedded HDS flash streams', which I don't fully understand, but https://n1njahacks.wordpress.com/2015/01/29/how-to-save-hds-flash-streams-from-any-web-page/ is a useful link which features a php program that one can use to assemble of all the different f4f files.

However, this requires to get a hold on the 'manifest', which in my case looks like http://adaptiv.wdr.de/z/medp/ww/fsk0/112/1120663/,1120663_12909203,1120663_12908906,1120663_12908907,.mp4.csmil/manifest.f4m?g=OREKBERCWCLP&hdcore=3.9.0&plugin=aasp-3.9.0.19.11

All but one of its parts can be grabbed from the source codes of different web pages by means of WWW:Mechanize conveniently. The missing piece is the 'g=OREKBERCWCLP', which is newly generated upon each run of the video by clicking on the arrow. Therefore the only thought that I had to find the piece programatically was to run the embedded video (but how??) and then use something like ethereal of tcpdump.