Nico, it may really be an issue with js. The only (apparently) broken code is displayed in the console. "Show Source Code" reveals that all data is at least delivered to the right places. So I guess I'll have to examine the js.
Thanks a lot guys!
--- Edit: It was something entirly different: UTF-8 coding instead of UTF-8 w/o BOM. Using Chrome (and its console) revealed some (invisble) code, that scrambled the header. Firebug didn't show it.
Thank you guys again, I learned a lot about Processwire on the way.
Cheers, Phil
Phil's post in header-data gets transfered into body was marked as the answer