Device on LAN3 not accessible

Since few days I have an issue with one of my devices connected to TO over LAN3. I have tried to exchange ethernet cable -> issue remains. I have also tried to use a different LAN connector and then everything is working fine again. That might indicate there is either something wrong with the configuration (I have not modified any settings in last month or so) or it is a hardware problem.
Would anyone have a hint where to check/what to look for, please?

My kernel Version is 4.14.162, fw TurrisOS 4.0.5 ab9d1bf / LuCI branch (git-20.016.50399-e1df28d)

Here’s a sample from the log:

Feb 29 22:25:16 turris kernel: [604072.611380] mv88e6085 f1072004.mdio-mii:10 lan3: Link is Up - 100Mbps/Full - flow control rx/tx
Feb 29 22:25:16 turris kernel: [604072.620221] br-lan: port 4(lan3) entered blocking state
Feb 29 22:25:16 turris kernel: [604072.625557] br-lan: port 4(lan3) entered forwarding state
Feb 29 21:25:16 turris netifd: Network device 'lan3' link is up
Feb 29 21:25:21 turris netifd: Network device 'lan3' link is down
Feb 29 22:25:21 turris kernel: [604078.399628] mv88e6085 f1072004.mdio-mii:10 lan3: Link is Down
Feb 29 22:25:21 turris kernel: [604078.405527] br-lan: port 4(lan3) entered disabled state

Is this issue specific to only one of the devices, which you have? Did you try to connect a different device to the LAN 3 port to see if the issue appears with another device as well?

Did you try to plug this one device to other LAN ports as well? I’m wondering if the issue remains.

This helps us to know if it there is something wrong with the one device or there is something wrong with the LAN3 port.

Yes, it is specific only to one device - which is a QNAP NAS which gets a fixed IP address from TO.
I have tried to plug the NAS into LAN4 - with same result. A device that was on LAN4 before was working ok, same when plugged into LAN3.

Here’s again a (redacted) system log after i have moved the NAS from LAN3 to LAN4:

Mar  1 23:29:51 turris kernel: [28003.259541] mv88e6085 f1072004.mdio-mii:10 lan4: Link is Up - 100Mbps/Full - flow control rx/tx
Mar  1 23:29:51 turris kernel: [28003.268297] br-lan: port 5(lan4) entered blocking state
Mar  1 23:29:51 turris kernel: [28003.273540] br-lan: port 5(lan4) entered forwarding state
Mar  1 22:29:51 turris netifd: Network device 'lan4' link is up
Mar  1 23:29:55 turris dnsmasq-dhcp[6511]: DHCPREQUEST(br-lan) 192.168.1.5 01.23:45:67:89:ab 
Mar  1 23:29:55 turris dnsmasq-dhcp[6511]: DHCPACK(br-lan) 192.168.1.5 01.23:45:67:89:ab qnap
Mar  1 22:29:55 turris 99-dhcp_host_domain_ng.py: Add_lease, hostname check failed
Mar  1 22:29:55 turris 99-dhcp_host_domain_ng.py: DHCP unknown update operation
<redacted>
Mar  1 22:29:55 turris kresd[7495]: > hints.del('qnap.lan')
Mar  1 22:29:55 turris kresd[7495]: [result] => true
<redacted>
Mar  1 22:29:55 turris kresd[7495]: > hints.del('qnap-old.lan')
Mar  1 22:29:55 turris kresd[7495]: [result] => true
<redacted>
Mar  1 22:29:55 turris kresd[7495]: > hints.del('m-t-BB9300.lan')
Mar  1 22:29:55 turris kresd[7495]: [result] => false
<redacted>
Mar  1 22:29:55 turris 99-dhcp_host_domain_ng.py: Refresh kresd leases
<redacted>
Mar  1 22:29:55 turris kresd[7495]: > hints.del('qnap_lan2.lan')
Mar  1 22:29:55 turris kresd[7495]: [result] => true
Mar  1 22:29:55 turris kresd[7495]: 
Mar  1 22:29:55 turris kresd[7495]: > hints.del('qnap.lan')
Mar  1 22:29:55 turris kresd[7495]: [result] => false
Mar  1 22:29:55 turris kresd[7495]: 
Mar  1 22:29:55 turris kresd[7495]: > hints.add_hosts('/tmp/kresd/hints.tmp')
Mar  1 22:29:55 turris kresd[7495]: [result] => true
Mar  1 22:29:55 turris kresd[7495]: 
Mar  1 22:29:57 turris netifd: Network device 'lan4' link is down
Mar  1 23:29:57 turris kernel: [28009.743591] mv88e6085 f1072004.mdio-mii:10 lan4: Link is Down
Mar  1 23:29:57 turris kernel: [28009.749399] br-lan: port 5(lan4) entered disabled state

A quick follow-up question: I just noticed that some of the items in the system log have a timestamp off by 1 hour.
How could that happen? Looking at the settings under luci/system tab, all seem to be set correctly (I see local time shown there matching current time).

Mar  6 16:48:50 turris kresd[7469]: 
Mar  6 17:48:53 turris kernel: [29158.425514] mv88e6085 f1072004.mdio-mii:10 lan4: Link is Down
Mar  6 17:48:53 turris kernel: [29158.431893] br-lan: port 5(lan4) entered disabled state
Mar  6 16:48:53 turris netifd: Network device 'lan4' link is down
Mar  6 16:49:01 turris /usr/sbin/cron[14011]: (root) CMD (/usr/bin/rainbow_button_sync.sh)
Mar  6 17:49:17 turris kernel: [29182.899734] mv88e6085 f1072004.mdio-mii:10 lan4: Link is Up - 100Mbps/Full - flow control rx/tx
Mar  6 17:49:17 turris kernel: [29182.908549] br-lan: port 5(lan4) entered blocking state
Mar  6 17:49:17 turris kernel: [29182.913799] br-lan: port 5(lan4) entered forwarding state
Mar  6 16:49:17 turris netifd: Network device 'lan4' link is up
Mar  6 17:49:23 turris kernel: [29189.072749] mv88e6085 f1072004.mdio-mii:10 lan4: Link is Down
Mar  6 17:49:23 turris kernel: [29189.078895] br-lan: port 5(lan4) entered disabled state
Mar  6 16:49:23 turris netifd: Network device 'lan4' link is down