It seems that something is strange with port1, which explains, why I couldn't access it via vlan100. Someone else was having the same issues with port1: Support for RTL838x based managed switches - #355 by sven
So the solution is:
Add any other port to the LAN vlan via the console (edit /etc/config/network and afterwards restart the service with "/etc/init.d/network restart") and you will be able to access Lucy normally.