OMV6 on J1900 motherboard: regular ethernet disconnection

Hi all, a curious case: OMV6 machine on q1800 motherboard (J1900 processor), regularly updated.

The server is at my in-laws' house.

Yesterday I noticed, by chance, that the server was losing connection at regular and relatively close intervals (every 3-4 minutes), for 5-10 seconds, then it reconnects.

I ruled out it was an ethernet cable issue, so the cause could be either the modem or the server's network card.

I tend to exclude the modem, because the desktop pc (windows 10) connected to the same network works well and is stable in navigation, so everything leads me to the server's network card.

These are lan specifics on the server:

Interface information enp2s0:
=============================
Settings for enp2s0:
Supported ports: [ TP MII ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Supported pause frame use: Symmetric Receive-only
Supports auto-negotiation: Yes
Supported FEC modes: Not reported
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Full
Advertised pause frame use: Symmetric Receive-only
Advertised auto-negotiation: No
Advertised FEC modes: Not reported
Speed: 1000Mb/s
Duplex: Full
Auto-negotiation: off
master-slave cfg: preferred slave
master-slave status: slave
Port: Twisted Pair
PHYAD: 0
Transceiver: external
MDI-X: Unknown
Supports Wake-on: pumbg
Wake-on: d
Link detected: yes
--------------------------------------------------------------------------------
Driver information enp2s0:
==========================
driver: r8169
version: 6.1.0-0.deb11.5-amd64
firmware-version: rtl8168g-2_0.0.1 02/06/13
expansion-rom-version:
bus-info: 0000:02:00.0
supports-statistics: yes
supports-test: no
supports-eeprom-access: no
supports-register-dump: yes
supports-priv-flags: no

I enclose also logfiles.

messages(2).txtsyslog(2).txt

in the last line of the "messages" log, my (failed) attempt to limit the problem is detected by using the command:

ethtool -s enp2s0 speed 1000 duplex full autoneg off

It is very frustrating, I would really appreciate if someone could help me.

Thank u very much.

Sorry for the delay,

no motherboard has a single NIC.

It is very curious.

It almost seems that some user connected to the electrical system does not comply with the electromagnetic compatibility standards.