Unstable RED tunnel and lots of martian source entries in kernel.log


I tried to set up an UTM-to-UTM RED Firewall Server tunnel (no legacy).
Sometimes I get packetloss for around 10-20 seconds. Sometimes the tunnel doesn't even come up.
And I noticed lots of martian source entries in kernel.log after enabling this RED tunnel:

2023:01:25-14:07:21 MYUTMNAME kernel: [6324855.992965] IPv4: martian source RED-SERVER-IP from MY-WAN-IP, on dev eth5
2023:01:25-14:07:21 MYUTMNAME kernel: [6324855.993049] ll header: 00000000: ff ff ff ff ff ff XX XX XX XX XX XX XX XX ...........D..

(Loglines anonymized)

But if I use RED Firewall Server Legacy instead of RED Firewall Server it works without any packetloss/problems.
Also an IPSec tunnel works without any problems for weeks.

(I have multiple WAN interfaces facing the router of my company ISP)

What could be the problem here?


Typo in subject
[edited by: UTMaddict at 2:12 PM (GMT -8) on 25 Jan 2023]