So do I have to configure a IPv6 adress, even if I don't use/need it, on the LAN interface or is this a bug?
You need to configure an IPv6 address on the LAN interface, even if you don't use it (disable RA if you don't want clients on LAN to obtain an address automatically) - due to the way m0n0wall was initially designed, various pieces of code assume that there is always an IP address configured on LAN, and the same applies to IPv6.
I've added a note that explains this on the System: Advanced page (now in the repository), so hopefully this will be more clear in the next version.