Hannes

I do ran into the same problem today with my rock-3a running
    Armbian 24.2.1 Bookworm with Linux 6.6.16-current-rockchip64

It occurred after re-boot of the system because of the banner the message "Please reboot system do to kernel-updated" appeared. After that ethernet (was named "end0") did not come up anymore. When activating using "nmtui" the message comes:
"Activation failed. Device could not be prepared for activation ..." (freely translated from German language).

journalctl -x gives the following:
(armbian-monitor -u already performed)
Mar 14 10:51:01 cloud kernel: rk_gmac-dwmac fe010000.ethernet end0: __stmmac_open: Cannot attach to PHY (error: -19) Mar 14 10:51:01 cloud NetworkManager[745]: <info>  [1710409861.7565] device (end0): state change: config -> failed (reason 'config-failed', sys-iface-state: 'managed') Mar 14 10:51:01 cloud NetworkManager[745]: <warn>  [1710409861.7579] device (end0): Activation: failed for connection 'Ethernet' Mar 14 10:51:01 cloud NetworkManager[745]: <info>  [1710409861.7588] device (end0): state change: failed -> disconnected (reason 'none', sys-iface-state: 'managed')
After all I updated the bootloader (using armbian-config) and it luckily resolved the problem ...

Thx for reading this. Hope it helps others ...