I found a hint in the MS KB:http://support.microsoft.com/kb/929852/en-us
About the 6to4 tunneling protocol
By default, the 6to4 tunneling protocol is enabled in Windows 7, Windows Vista, Windows Server 2008 R2, and Windows Server 2008 when an interface is assigned a public IPv4 address (that is, an IPv4 address that is not in the ranges 10.0.0.0/8, 172.16.0.0/12, or 192.168.0.0/16). 6to4 automatically assigns an IPv6 address to the 6to4 tunneling interface for each such address that is assigned, and 6to4 will dynamically register these IPv6 addresses on the assigned DNS server. If this behavior is not desired, we recommend that you disable IPv6 tunnel interfaces on the affected hosts.
They say, if you use a non private ipv4 adress than the 6to4 adapter start working. Why I not fully understand. And we use non private IPv4 Adresse for our iSCSI Interfaces.
I now disabled the 6to4 adapter in the device manager (it is hidden), this stops generating errors. This is ofcouse not a solution but Microsoft says it is okay (see quote above)