Thanks. I hope the installation worked - I only have Focal now, and the headers package in that folder was asking for a dependency from Eoan which I don't have.

Still, I managed to boot the system, and unfortunately the behaviour is the same.

Aug  1 12:04:56 rpi3 kernel: [  117.985005] ------------[ cut here ]------------
Aug  1 12:04:56 rpi3 kernel: [  117.985053] NETDEV WATCHDOG: eth0 (lan78xx): transmit queue 0 timed out
Aug  1 12:04:56 rpi3 kernel: [  117.985163] WARNING: CPU: 1 PID: 0 at /home/hwang4/work/mainline/build/eoan-rpi/ubuntu-eoan/net/sched/sch_generic.c:448 dev_watchdog+0x370/0x378
Aug  1 12:04:56 rpi3 kernel: [  117.985168] Modules linked in: sit tunnel4 ip_tunnel bridge stp llc ip6table_filter ip6_tables xt_tcpudp xt_conntrack nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 iptable_filter bpfilter dm_multipath scsi_dh_rdac scsi_dh_emc scsi_dh_alua btsdio bluetooth ecdh_generic ecc brcmfmac brcmutil cfg80211 input_leds bcm2835_v4l2(CE) bcm2835_mmal_vchiq(CE) raspberrypi_hwmon vc_sm_cma(CE) raspberrypi_cpufreq v4l2_common videobuf2_vmalloc videobuf2_memops videobuf2_v4l2 videobuf2_common videodev mc uio_pdrv_genirq uio sch_fq_codel drm ip_tables x_tables autofs4 btrfs zstd_compress raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor xor_neon raid6_pq libcrc32c raid1 raid0 multipath linear hid_generic usbhid sdhci_iproc spidev phy_generic crct10dif_ce fixed aes_neon_bs aes_neon_blk crypto_simd cryptd aes_arm64
Aug  1 12:04:56 rpi3 kernel: [  117.985309] CPU: 1 PID: 0 Comm: swapper/1 Tainted: G         C  E     5.3.0-1030-raspi2 #32+testpreempt
Aug  1 12:04:56 rpi3 kernel: [  117.985311] Hardware name: Raspberry Pi 3 Model B Plus Rev 1.3 (DT)
Aug  1 12:04:56 rpi3 kernel: [  117.985316] pstate: 80400005 (Nzcv daif +PAN -UAO)
Aug  1 12:04:56 rpi3 kernel: [  117.985320] pc : dev_watchdog+0x370/0x378
Aug  1 12:04:56 rpi3 kernel: [  117.985324] lr : dev_watchdog+0x370/0x378
Aug  1 12:04:56 rpi3 kernel: [  117.985326] sp : ffff00001000bd90
Aug  1 12:04:56 rpi3 kernel: [  117.985329] x29: ffff00001000bd90 x28: ffff567789c1abf0 
Aug  1 12:04:56 rpi3 kernel: [  117.985334] x27: ffffeeb8788d4480 x26: 00000000ffffffff 
Aug  1 12:04:56 rpi3 kernel: [  117.985338] x25: 0000000000000140 x24: ffffeeb86f5c8480 
Aug  1 12:04:56 rpi3 kernel: [  117.985342] x23: ffff56778a807000 x22: ffffeeb8788d445c 
Aug  1 12:04:56 rpi3 kernel: [  117.985346] x21: ffffeeb8788d4000 x20: ffffeeb8788d4480 
Aug  1 12:04:56 rpi3 kernel: [  117.985351] x19: 0000000000000000 x18: ffffffffffffffff 
Aug  1 12:04:56 rpi3 kernel: [  117.985355] x17: 0000000000000000 x16: 0000000000000000 
Aug  1 12:04:56 rpi3 kernel: [  117.985359] x15: ffff56778a809708 x14: ffff56778a9418e0 
Aug  1 12:04:56 rpi3 kernel: [  117.985363] x13: ffff56778a941533 x12: ffff56778a82e000 
Aug  1 12:04:56 rpi3 kernel: [  117.985367] x11: 0000000000000000 x10: ffff56778a940000 
Aug  1 12:04:56 rpi3 kernel: [  117.985372] x9 : 0000000000000000 x8 : ffff56778a94b8ab 
Aug  1 12:04:56 rpi3 kernel: [  117.985376] x7 : 0000000000000000 x6 : 0000000000000002 
Aug  1 12:04:56 rpi3 kernel: [  117.985379] x5 : 0000000000000000 x4 : 0000000000000002 
Aug  1 12:04:56 rpi3 kernel: [  117.985385] x3 : 0000000000000000 x2 : 0000000000000004 
Aug  1 12:04:56 rpi3 kernel: [  117.985389] x1 : 2668adad17146c00 x0 : 0000000000000000 
Aug  1 12:04:56 rpi3 kernel: [  117.985394] Call trace:
Aug  1 12:04:56 rpi3 kernel: [  117.985400]  dev_watchdog+0x370/0x378
Aug  1 12:04:56 rpi3 kernel: [  117.985407]  call_timer_fn+0x38/0x1e8
Aug  1 12:04:56 rpi3 kernel: [  117.985412]  run_timer_softirq+0x518/0x550
Aug  1 12:04:56 rpi3 kernel: [  117.985417]  __do_softirq+0x178/0x428
Aug  1 12:04:56 rpi3 kernel: [  117.985422]  irq_exit+0xf4/0x100
Aug  1 12:04:56 rpi3 kernel: [  117.985429]  __handle_domain_irq+0x90/0xf8
Aug  1 12:04:56 rpi3 kernel: [  117.985433]  bcm2836_arm_irqchip_handle_irq+0x68/0xd0
Aug  1 12:04:56 rpi3 kernel: [  117.985436]  el1_irq+0x108/0x200
Aug  1 12:04:56 rpi3 kernel: [  117.985443]  arch_cpu_idle+0x30/0x228
Aug  1 12:04:56 rpi3 kernel: [  117.985453]  default_idle_call+0x28/0x4c
Aug  1 12:04:56 rpi3 kernel: [  117.985458]  do_idle+0x1e0/0x2c8
Aug  1 12:04:56 rpi3 kernel: [  117.985464]  cpu_startup_entry+0x28/0x30
Aug  1 12:04:56 rpi3 kernel: [  117.985473]  secondary_start_kernel+0x184/0x250
Aug  1 12:04:56 rpi3 kernel: [  117.985476] ---[ end trace 6bb7d08c2e67083a ]---