No access to Ethernet card since 5.4.0-56-lowlatency

No LEDs mean no connection at the physical level. There should at least be an orange light. Have you tried a different ethernet cable?

What is the output of:

sudo lshw -C network

Thanks

There are no LEDs on, not even the orange one. The Ethernet cable is fine though, since I can access the Internet without problem when I boot using a kernel prior to 5.4.0-56-lowlatency.

dmesg | tail (under 5.4.0-56-lowlatency or later) gives the following output:

[ 42.131699] genirq: Flags mismatch irq 16. 00010080 (enp4s0) vs. 00002080 (uhci_hcd:usb3)

sudo lshw -C network (under 5.4.0-56-lowlatency or later) gives this output:

 *-network DESACTIVADO
       descripción: Ethernet interface
       producto: RTL8111/8168/8411 PCI Express Gigabit Ethernet Controller
       fabricante: Realtek Semiconductor Co., Ltd.
       id físico: 0
       información del bus: pci@0000:04:00.0
       nombre lógico: enp4s0
       versión: 01
       serie: 00:1f:d0:5a:81:b9
       anchura: 64 bits
       reloj: 33MHz
       capacidades: pm vpd msi pciexpress bus_master cap_list rom ethernet physical
       configuración: broadcast=yes driver=r8169 latency=0 link=no multicast=yes
       recursos: irq:16 ioport:d000(size=256) memoria:f8000000-f8000fff memoria:f7000000-f701ffff

OK, did you enable the interface?

I'm not sure how to do that. Also, I'm not sure if it's possible. The interface works fine in every kernel prior to version 5.4.0-56-lowlatency (I'm using 5.4.0-42-lowlatency now to be able to connect to the Internet) but it doesn't in 5.4.0-56-lowlatency and later (the update to 5.4.0-56-lowlatency some days ago made the problem appear). I'm not doing anything else apart from booting with different kernels. Today there was an automatic update to kernel version 5.4.0-58-lowlatency which I thought might solve the problem, but didn't. As the output of dmesg reveals, there seems to be a conflict with IRQs, which disappears when booting with an earlier kernel.

This bug might be related: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1906655

https://wiki.ubuntu.com/Apport
 Enter the following command in a terminal window: apport-collect 1906655
And add this to the comment section of the bug report that affects you.
This will collect debugging information for the bug report.

Thank you, just did it. I executed the command while using the 5.4.0-42-lowlatency version kernel, since it's the latest version that works correctly with the Ethernet card and allows an Internet connection.

The recent kernel update to version 5.4.0-59 solved the problem. The Ethernet works correctly. Thanks for all the answers and comments.