The signal strength is excelent (maximum green). Direct wired connection test results are confirm what my provider is offering me: 59.9 Mpbs download and 5.74 Mbps download.
Transfering a file of 1000 Mb from a wired computer to the wireless machine gives good results as well.
Today i spoke to the local helpdesk in the Netherlands and they told me that the new high speed modems are using new protocols. Because of this they are working on a new firmware, which should solve this problem. Some strange voice in the back of my head is telling me that this won't be solved by upgrading to a new firmware, but i hope that they are right!