@olivierlambert Just produced another reboot. I'm closing in on the way to replicate this issue.

[ 334371.865769]  ALERT: BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
[ 334371.865787]   INFO: PGD 2250ed067 P4D 2250ed067 PUD 228c9f067 PMD 0
[ 334371.865803]   WARN: Oops: 0000 [#1] SMP NOPTI
[ 334371.865810]   WARN: CPU: 9 PID: 57 Comm: ksoftirqd/9 Tainted: G           O      4.19.0+1 #1
[ 334371.865818]   WARN: Hardware name: Dell Inc. PowerEdge R720/0C4Y3R, BIOS 2.9.0 12/06/2019
[ 334371.865832]   WARN: RIP: e030:skb_copy_ubufs+0x19c/0x5f0
[ 334371.865839]   WARN: Code: 90 cc 00 00 00 48 03 90 d0 00 00 00 48 63 44 24 40 48 83 c0 03 48 c1 e0 04 48 01 d0 48 89 18 c7 40 08 00 00 00 00 44 89 78 0c <48> 8b 43 08 a8 01 0f 85 3f 04 00 00 48 8b 44 24 30 48 83 78 20 ff
[ 334371.865858]   WARN: RSP: e02b:ffffc9004026b6f8 EFLAGS: 00010282
[ 334371.865864]   WARN: RAX: ffff888099621ae0 RBX: 0000000000000000 RCX: 00000000000000c0
[ 334371.865873]   WARN: RDX: ffff888099621ac0 RSI: ffff888099621ac0 RDI: ffffea00031da880
[ 334371.865881]   WARN: RBP: 0000000000000000 R08: ffff888099621a00 R09: ffff8881f0d43e98
[ 334371.865890]   WARN: R10: ffffc9004026b8b0 R11: 0000000000000000 R12: ffff888096e61c00
[ 334371.865898]   WARN: R13: 0000000000000000 R14: ffff88822b867a80 R15: 0000000000000000
[ 334371.865918]   WARN: FS:  0000000000000000(0000) GS:ffff88822d440000(0000) knlGS:0000000000000000
[ 334371.865927]   WARN: CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 334371.865935]   WARN: CR2: 0000000000000008 CR3: 00000002281aa000 CR4: 0000000000040660
[ 334371.865949]   WARN: Call Trace:
[ 334371.865958]   WARN:  skb_clone+0x71/0xa0
[ 334371.865968]   WARN:  do_execute_actions+0x4ec/0x1750 [openvswitch]
[ 334371.865978]   WARN:  ? ovs_dp_process_packet+0x7d/0x110 [openvswitch]
[ 334371.865988]   WARN:  ? ovs_vport_receive+0x6e/0xd0 [openvswitch]
[ 334371.865997]   WARN:  ? arch_local_irq_restore+0x5/0x10
[ 334371.866005]   WARN:  ? get_page_from_freelist+0xa4f/0xf00
[ 334371.866012]   WARN:  ? arch_local_irq_restore+0x5/0x10
[ 334371.866020]   WARN:  ? get_page_from_freelist+0xa4f/0xf00
[ 334371.866031]   WARN:  ovs_execute_actions+0x47/0x120 [openvswitch]
[ 334371.866040]   WARN:  ovs_dp_process_packet+0x7d/0x110 [openvswitch]
[ 334371.866050]   WARN:  ? key_extract+0xa53/0xd60 [openvswitch]
[ 334371.866058]   WARN:  ovs_vport_receive+0x6e/0xd0 [openvswitch]
[ 334371.866066]   WARN:  ? __alloc_skb+0x4e/0x270
[ 334371.866075]   WARN:  ? notify_remote_via_irq+0x4a/0x70
[ 334371.866085]   WARN:  ? __raw_callee_save_xen_vcpu_stolen+0x11/0x20
[ 334371.866091]   WARN:  ? __alloc_skb+0x76/0x270
[ 334371.866100]   WARN:  ? arch_local_irq_restore+0x5/0x10
[ 334371.866108]   WARN:  ? __slab_alloc.constprop.81+0x42/0x4e
[ 334371.866114]   WARN:  ? __alloc_skb+0x4e/0x270
[ 334371.866120]   WARN:  ? __kmalloc_track_caller+0x58/0x200
[ 334371.866127]   WARN:  ? __slab_alloc.constprop.81+0x42/0x4e
[ 334371.866136]   WARN:  ? __kmalloc_reserve.isra.48+0x29/0x70
[ 334371.866146]   WARN:  netdev_frame_hook+0x105/0x180 [openvswitch]
[ 334371.866154]   WARN:  __netif_receive_skb_core+0x211/0xb30
[ 334371.866163]   WARN:  __netif_receive_skb_one_core+0x36/0x70
[ 334371.866170]   WARN:  netif_receive_skb_internal+0x34/0xe0
[ 334371.866179]   WARN:  xenvif_tx_action+0x55c/0x990
[ 334371.866187]   WARN:  xenvif_poll+0x27/0x70
[ 334371.866193]   WARN:  net_rx_action+0x2a5/0x3e0
[ 334371.866200]   WARN:  __do_softirq+0xd1/0x28c
[ 334371.866208]   WARN:  run_ksoftirqd+0x26/0x40
[ 334371.866215]   WARN:  smpboot_thread_fn+0x10e/0x160
[ 334371.866223]   WARN:  kthread+0xf8/0x130
[ 334371.866229]   WARN:  ? sort_range+0x20/0x20
[ 334371.866235]   WARN:  ? kthread_bind+0x10/0x10
[ 334371.866242]   WARN:  ret_from_fork+0x35/0x40
[ 334371.866250]   WARN: Modules linked in: tun bnx2fc(O) cnic(O) uio fcoe libfcoe libfc scsi_transport_fc openvswitch nsh nf_nat_ipv6 nf_nat_ipv4 nf_conncount nf_nat 8021q garp mrp stp llc dm_multipath ipt_REJECT nf_reject_ipv4 xt_tcpu$
[ 334371.866374]   WARN:  scsi_mod efivarfs ipv6 crc_ccitt
[ 334371.866384]   WARN: CR2: 0000000000000008
[ 334371.866396]   WARN: ---[ end trace 8b74661a79be8268 ]---
[ 334371.868712]   WARN: RIP: e030:skb_copy_ubufs+0x19c/0x5f0
[ 334371.868721]   WARN: Code: 90 cc 00 00 00 48 03 90 d0 00 00 00 48 63 44 24 40 48 83 c0 03 48 c1 e0 04 48 01 d0 48 89 18 c7 40 08 00 00 00 00 44 89 78 0c <48> 8b 43 08 a8 01 0f 85 3f 04 00 00 48 8b 44 24 30 48 83 78 20 ff
[ 334371.868740]   WARN: RSP: e02b:ffffc9004026b6f8 EFLAGS: 00010282
[ 334371.868748]   WARN: RAX: ffff888099621ae0 RBX: 0000000000000000 RCX: 00000000000000c0
[ 334371.868759]   WARN: RDX: ffff888099621ac0 RSI: ffff888099621ac0 RDI: ffffea00031da880
[ 334371.868769]   WARN: RBP: 0000000000000000 R08: ffff888099621a00 R09: ffff8881f0d43e98
[ 334371.868778]   WARN: R10: ffffc9004026b8b0 R11: 0000000000000000 R12: ffff888096e61c00
[ 334371.868788]   WARN: R13: 0000000000000000 R14: ffff88822b867a80 R15: 0000000000000000
[ 334371.868805]   WARN: FS:  0000000000000000(0000) GS:ffff88822d440000(0000) knlGS:0000000000000000
[ 334371.868815]   WARN: CS:  e033 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 334371.868823]   WARN: CR2: 0000000000000008 CR3: 00000002281aa000 CR4: 0000000000040660
[ 334371.868837]  EMERG: Kernel panic - not syncing: Fatal exception in interrupt

I found the problem.
I am using OPNsense and forgot to disable TX checksum offloading. Very interesting that this checksum offloading caused catastrophic network disruptions on a Realtek nic, but no noticeable performance hit on Intel nics. This was an old host that featured a Realtek card. All my recent hosts that I use have only Intel nics. That is why I forgot about the whole offloading thing.

Thanks for the tips.

Best wishes to the whole community!