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

Hello,

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?

Regards
UTMaddict