How to troubleshoot SFP+ connection flapping between two linux servers?

I'm trying to set up an SFP+ connection between two servers. The port keeps flapping.

root@routeur:~# dmesg|tail
[20266.254806] ixgbe 0000:04:00.1 eno8: NIC Link is Up 10 Gbps, Flow Control: RX/TX
[20266.259946] ixgbe 0000:04:00.1 eno8: NIC Link is Down
[20291.341648] ixgbe 0000:04:00.1 eno8: NIC Link is Up 10 Gbps, Flow Control: RX/TX
[20291.445598] ixgbe 0000:04:00.1 eno8: NIC Link is Down
[20339.204093] ixgbe 0000:04:00.1 eno8: NIC Link is Up 10 Gbps, Flow Control: RX/TX
[20339.208501] ixgbe 0000:04:00.1 eno8: NIC Link is Down

root@routeur:~# ethtool eno8
Settings for eno8:
        Supported ports: [ FIBRE ]
        Supported link modes:   10000baseT/Full
        Supported pause frame use: Symmetric
        Supports auto-negotiation: No
        Supported FEC modes: Not reported
        Advertised link modes:  10000baseT/Full
        Advertised pause frame use: Symmetric
        Advertised auto-negotiation: No
        Advertised FEC modes: Not reported
        Speed: Unknown!
        Duplex: Unknown! (255)
        Port: FIBRE
        PHYAD: 0
        Transceiver: internal
        Auto-negotiation: off
        Supports Wake-on: d
        Wake-on: d
        Current message level: 0x00000007 (7)
                               drv probe link
        Link detected: no

root@routeur:~# lshw -C network -businfo | grep eno8
pci@0000:04:00.1  eno8       network        Ethernet Connection X552 10 GbE SFP+

root@client2:~# lshw -C network -businfo | grep enp8s0
pci@0000:08:00.0  enp8s0      network        MT26448 [ConnectX EN 10GigE, PCIe 2.0 5GT/s]

What should i be looking at ?

router and client are both running ProxMox (Debian 10)

root@routeur:~# cat /etc/network/interfaces
auto lo
iface lo inet loopback

[...]
#SFP
auto eno8
iface eno8 inet static
        address 10.A.B.C/30

root@client2:~# cat /etc/network/interfaces
auto lo
iface lo inet loopback

[...]
auto enp8s0
iface enp8s0 inet static
        address 10.X.Y.Z/30
        gateway 10.A.B.C

routeur is a Supermicro SYS-5018D-FN8T client2 is a Dell PowerEdge R610