Since I had to fiddle with the settings at the router here is what finally worked for me:
RFC 2516 PPPoE
Multiplexing: LLC
QoS Type: UBR
PCR: default (4000)
SCR: default (4000)
VPI: 1
VCI: 32
DSL Modulation: ADSL2+
Username: 1und1/xxxx-yyy@online.de
Password: [your password]
Note: This works for ADSL based connections. VDSL is a different topic.