systemd-networkd – no arp replies

After completely switching my Armbian forky machine to systemd-networkd my OpenWrt router doesn't reply to arp requests anymore.

tcpdump shows the requests on the forky machine as well as on the router but there is no reply. In consequence the arp cache doesn't gets no entry and routing/forwarding to the router fails.

What I have done so far:

• ipv4 routes checked positiv
• static ipv4 address is set
• ipv6 works perfectly
• used tcpdump to check arp requests (positiv) and replies (negativ)
• apr-scan can send arp requests and even receive replies!!! Arp cache is updated but ping still fails
• ICMP and TCP connections in the network works but forwarding to the internet fails
• there is no specific firewall rule for the forky machine
• other devices (Mac, a lot of smart home devices, office laptop, …) work perfectly
• sysctl *arp_ignore is set to 0 on the router

Network file in /etc/systemd/network (all other systemd-networkd configurations removed):

[Match]
Name=end0

[Network]
Address=192.168.2.6/24
Gateway=192.168.2.1
#DefaultRouteOnDevice=yes
DNS=192.168.2.6
DNS=192.168.2.7

Domains=iot.home guest.home untrusted.home

DHCP=ipv6
LinkLocalAddressing=yes
LLDP=yes
EmitLLDP=yes
IPv6AcceptRA=yes
IPv6SendRA=no

[IPv6AcceptRA]
# Required to properly accept Prefix Delegation
DHCPv6Client=always
UseOnLinkPrefix=yes
UseAutonomousPrefix=yes

Anybody out there who has any idea how to proceed?