I finally now got a 3G/UMTS (HSDPA) internet flatrate at the german provider 1und1/1&1. There seem to be two setups that work for the connection and that are accounted correctly (as flatrate use). The first one is a 1und1 special setup that only gives you limited internet access (e.g. you cannot ping any machine in the internet). The second one – which i preferr – is the usual setup for german D2/Vodafone network:
APN: web.vodafone.de
Username: vodafone
Password: vodafone
Just for completeness the 1und1 special configs are:
APN: mail.partner.de
Username: D2
Password: Web
You can also find all those data on the providers homepages, but you have to dig quite deep until you find them. So hopefully this is of use for some future users.