Display ethernet doesn't find IP, but does work with manual IP

My MacBook Pro is connected to my thunderbolt display. Recently the display ethernet stopped working. When I set the setting on 'Using DHCP' it doesn't find an IP address. I can select 'Using DHCP with manual address' and choose a random IP address and it works fine. The wifi also works fine (so that does find an IP address using DHCP...).

Any ideas how to solve this? Is it a mac or a router problem?

Thanks in advance,

Rugen Heidbuchel