I finally found the problem. Indeed it was a routing issue. Per default the Linux kernel does not accept routing information from ICMPv6 messages, so I put
echo 128 > /proc/sys/net/ipv6/conf/all/accept_ra_rt_info_max_plen
Optionally I could set the route via the wlan-interface manually by using:
route -6 add <prefix of WAN interface>/64 gw <address of wlan0 interface>
on the client situated in the LAN