As I got from the internet, it sporadically appears and disappears, but could not find any explanations as to why and when it happens.
Personally, I started paying attention to this message after a disk full event in a skillsbench/benchflow setup (with Ubuntu 22.04 on the host) which for some time was broken, and then in some miraculous way restored.
I attach the full result of the sudo journalctl -u docker --since "yesterday" command after running a single skillsbench task. This run should build a new image based on an existing image, then execute the container and try do solve the task inside it. NB: Before running the task the output of this command was absolutely empty.
Sep 14 15:21:27 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:27.915433624+08:00" level=info msg="sbJoin: gwep4 ''-\>'d26d1becfbe5', gwep6 ''-\>''"
Sep 14 15:21:30 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:30.826430515+08:00" level=warning msg="failed to read oom_kill event" error="open /sys/fs/cgroup/system.slice:docker:honkhba31inybd5zsaj7ij992/memory.events: no such file or directory" span="\[ 4/11\] RUN pip3 install --break-system-packages numpy==1.26.4 pandas==2.2.2 pyyaml==6.0.1 matplotlib==3.8.4" spanID=958186f9afaf2de1 traceID=056e0fcdc092cddc5089025ee6eae905
Sep 14 15:21:30 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:30.902477576+08:00" level=error msg=/moby.buildkit.v1.Control/Solve error="rpc error: code = Canceled desc = context canceled" spanID=abe729f04ba9d6d6 traceID=056e0fcdc092cddc5089025ee6eae905
Sep 14 15:21:32 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:32.582300942+08:00" level=warning msg="healthcheck failed" actualDuration="411.476µs" error="Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=dbdd8824e727834b timeout=15s traceID=056e0fcdc092cddc5089025ee6eae905
Sep 14 15:21:32 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:32.613324480+08:00" level=warning msg="healthcheck failed" actualDuration="255.034µs" error="Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=eee6956ff59f2e27 timeout=15s traceID=056e0fcdc092cddc5089025ee6eae905
Sep 14 15:21:36 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:36.671470825+08:00" level=info msg="sbJoin: gwep4 ''-\>'64014a2f2d68', gwep6 ''-\>''"
Sep 14 15:21:37 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:37.581879965+08:00" level=error msg="healthcheck failed fatally" error="session healthcheck failed fatally: Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=dbdd8824e727834b traceID=056e0fcdc092cddc5089025ee6eae905
Sep 14 15:21:37 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:37.613222733+08:00" level=error msg="healthcheck failed fatally" error="session healthcheck failed fatally: Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=eee6956ff59f2e27 traceID=056e0fcdc092cddc5089025ee6eae905
Sep 14 15:21:41 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:41.430801004+08:00" level=warning msg="healthcheck failed" actualDuration="399.895µs" error="Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=5e81e8c22e9863cc timeout=15s traceID=0a1a2e4554d48b694efcdceae8632f92
Sep 14 15:21:46 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:46.430245069+08:00" level=error msg="healthcheck failed fatally" error="session healthcheck failed fatally: Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=5e81e8c22e9863cc traceID=0a1a2e4554d48b694efcdceae8632f92
Sep 14 15:21:58 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:58.847565464+08:00" level=info msg="sbJoin: gwep4 ''-\>'7697f6d7fe71', gwep6 ''-\>''"
Sep 14 15:21:59 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:59.248433513+08:00" level=info msg="sbJoin: gwep4 ''-\>'41d9a17de351', gwep6 ''-\>''"
Sep 14 15:21:59 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:21:59.699121015+08:00" level=info msg="sbJoin: gwep4 ''-\>'d93651c45ea6', gwep6 ''-\>''"
Sep 14 15:22:02 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:02.265647480+08:00" level=info msg="failed to read ipv6 net.ipv6.conf.\<bridge\>.accept_ra" bridge=br-7dbb657bc2d2 syspath=/proc/sys/net/ipv6/conf/br-7dbb657bc2d2/accept_ra
Sep 14 15:22:02 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:02.521648909+08:00" level=info msg="sbJoin: gwep4 ''-\>'c09eb5d27756', gwep6 ''-\>''" eid=c09eb5d27756 ep=adaptive-cruise-control__03bdbc1a-main-1 net=adaptive-cruise-control__03bdbc1a_default nid=7dbb657bc2d2 spanID=e3e431435a68e2b9 traceID=c4e805e5440c4faf8f3f8f3809c8d8ed
Sep 14 15:22:06 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:06.462882078+08:00" level=warning msg="healthcheck failed" actualDuration="378.501µs" error="Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=7922839b674cb24c timeout=15s traceID=0a1a2e4554d48b694efcdceae8632f92
Sep 14 15:22:07 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:07.110473764+08:00" level=warning msg="healthcheck failed" actualDuration="243.595µs" error="Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=5e7b57a2ebf5dc9d timeout=15s traceID=c4e805e5440c4faf8f3f8f3809c8d8ed
Sep 14 15:22:07 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:07.140789096+08:00" level=warning msg="healthcheck failed" actualDuration="316.527µs" error="Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=470347e1cbc83684 timeout=15s traceID=c4e805e5440c4faf8f3f8f3809c8d8ed
Sep 14 15:22:11 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:11.462464005+08:00" level=error msg="healthcheck failed fatally" error="session healthcheck failed fatally: Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=7922839b674cb24c traceID=0a1a2e4554d48b694efcdceae8632f92
Sep 14 15:22:12 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:12.110600501+08:00" level=error msg="healthcheck failed fatally" error="session healthcheck failed fatally: Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=5e7b57a2ebf5dc9d traceID=c4e805e5440c4faf8f3f8f3809c8d8ed
Sep 14 15:22:12 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:22:12.140960836+08:00" level=error msg="healthcheck failed fatally" error="session healthcheck failed fatally: Unavailable: connection error: desc = \\"transport: Error while dialing: only one connection allowed\\"" spanID=470347e1cbc83684 traceID=c4e805e5440c4faf8f3f8f3809c8d8ed
Sep 14 15:36:30 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:36:30.584669748+08:00" level=info msg="received task-delete event from containerd" container=3e990fa30b0804d5a9b3d50caba2d034ab600d8fc9028a8a0cfe608441d1e646 module=libcontainerd namespace=moby topic=/tasks/delete type="\*events.TaskDelete"
Sep 14 15:36:30 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:36:30.844288655+08:00" level=error msg="Error setting up exec command in container 3e990fa30b0804d5a9b3d50caba2d034ab600d8fc9028a8a0cfe608441d1e646: container 3e990fa30b0804d5a9b3d50caba2d034ab600d8fc9028a8a0cfe608441d1e646 is not running" spanID=d51afe9d13c72c7f traceID=20cdae30aaa1d87a907c51838ece8467
Sep 14 15:36:31 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:36:31.093665924+08:00" level=info msg="sbJoin: gwep4 ''-\>'0138ab81e487', gwep6 ''-\>''" eid=0138ab81e487 ep=adaptive-cruise-control__03bdbc1a-main-1 net=adaptive-cruise-control__03bdbc1a_default nid=7dbb657bc2d2 spanID=3a712a56f1b15a65 traceID=7427960b29f43e18ba1300e52daf701d
Sep 14 15:36:55 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:36:55.938014641+08:00" level=info msg="Container failed to exit within 10s of signal 15 - using the force" container=3e990fa30b0804d5a9b3d50caba2d034ab600d8fc9028a8a0cfe608441d1e646 spanID=a2290aa10d3ed548 traceID=3343f7feef949b71ec29d5c10288d542
Sep 14 15:36:55 mscpphys0200001 dockerd\[1091036\]: time="2026-09-14T15:36:55.982745361+08:00" level=info msg="received task-delete event from containerd" container=3e990fa30b0804d5a9b3d50caba2d034ab600d8fc9028a8a0cfe608441d1e646 module=libcontainerd namespace=moby topic=/tasks/delete type="\*events.TaskDelete"
Here the situation is described from another (likely wrong) angle.
After the setup started working again I still have this message, so it looks like this message is rather a ‘warning’ than an ‘error’. But I am still very curious to learn what it means, why it occurs, and how safe it is.
Actually I am not sure anymore that my setup is healthy. Can this message be a sign of a race condition inside docker transport which prevents a certain system variable getting into my container in 40% of runs ?