Apologies, bit late getting to this, Covid-19 depression is hard I miss sunlight.
Firstly run an update on Armbian as there have been some issues in December with Ethernet drivers so you may be looking at either a needed update or an update that’s reset your Ethernet config.
Secondly check your network settings by running ethtool eth0 look for anything disabled or if the link mode isn’t showing 1000baseT/Full you can also test the system by forcing gigabit networking by the command ethtool -s eth0 speed 100 autoneg off followed by ifup eth0 to bring it back online. (This is not persistent after a reboot, instructions to make it persistent are online)
One of the error messages in your your system logs “could” indicate that the network hardware is damaged it may also indicate that the Ethernet settings are back to default.
Was there a thunderstorm the day or night before you started having problems?
Network Checks (General List)
Some basic network checks to go through if your still having issues:-
On the NFS server check the settings for the network adaptor, make sure jumbo frames are enabled and anything to do with either “Green Ethernet” or Power Saving is disabled. As you say you haven’t touched anything it may be as simple as an upgrade has reset your network adaptor back to default settings. This would be my first go-to as it happens a lot
Try the following in any order you like.
Check the network sockets and plugs, use an air duster on the sockets if you have one blow the dust out with a straw if you haven’t. Wipe any contacts on the plugs clean with a tissue and then blow or air dust them.
I have had good results in the past using WD40 Specialist Contact Cleaner plus Ethernet (RJ45) Dust Covers are a cheap and good option for any unused ports, though if you don’t have fingernails buy the ones that have “knobs” on them for easier removal.
Try the steps listed in this very good and easy to understand tutorial for nfsstat. from the nice people at RedHat as it will tie down if its a physical issue.
NB nfs-utils is an entware package so if you need it SSL into CoreELEC and issue the command installentware wait until its installed and rebooted then SSL back in and issue opkg install nfs-utils
Run the network cable your using through your fingers and feel for any nicks or punctures. If you feel any chuck it and buy a new one.
Reboot any network hardware including switches then check that the router / modem / switches etc are all running on the latest firmware.
Run a sanity check to make sure that you haven’t moved anything that could cause RFI interference near the network cable run router or switches. DECT Cordless phones are a big culprit for this along with some cheap and not cheerful WIFI controlled lightbulbs.