PXE boot fail with kernel panic: Unable to mount root fs - linux-kernel

Question

I am having an issue on some servers since some time, and fail to find the issue.
These are x86_64 server, with Intel Xeon, configured to boot in UEFI over network, through an iPXE rom.
Kernel and initramfs are the ones from Centos 8 (tried 8.0 and 8.2).
But when booting, I always end up with (on every servers, so should not be related to an hardware failure):
[ 5.542304] hid-generic 0003:0557:2221.0002: input,hidraw1: USB HID v1.00 Keyboard [Winbond Electronics Corp Hermon USB hidmouse Device] on usb-0000:00:1a.0-1.3/input1
[ 5.599611] rtc_cmos 00:02: setting system clock to 2020-06-26 19:21:43 UTC (1593199303)
[ 5.620965] md: Waiting for all devices to be available before autodetect
[ 5.640580] md: If you don't use raid, use raid=noautodetect
[ 5.659949] md: Autodetecting RAID arrays.
[ 5.676869] md: autorun ...
[ 5.691838] md: ... autorun DONE.
[ 5.707883] List of all partitions:
[ 5.723667] No filesystem could mount root, tried:
[ 5.723667]
[ 5.754724] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 5.775237] CPU: 9 PID: 1 Comm: swapper/0 Not tainted 4.18.0-193.6.3.el8_2.x86_64 #1
[ 5.815778] Call Trace:
[ 5.830360] dump_stack+0x5c/0x80
[ 5.845920] panic+0xe7/0x2a9
[ 5.860995] mount_block_root+0x2c5/0x2e9
[ 5.877407] ? do_early_param+0x91/0x91
[ 5.892617] prepare_namespace+0x135/0x16b
[ 5.907676] kernel_init_freeable+0x22e/0x258
[ 5.922607] ? rest_init+0xaa/0xaa
[ 5.937398] kernel_init+0xa/0xff
[ 5.950991] ret_from_fork+0x35/0x40
[ 5.964572] Kernel Offset: 0x2ca00000 from 0xffffffff81000000 (relocation range: 0xffffffff80000000-0xffffffffbfffffff)
[ 6.000387] ---[ end Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) ]---
The iPXE boot script is:
#!ipxe
kernel http://10.10.0.1/vmlinuz-4.18.0-193.6.3.el8_2.x86_64 initrd=initramfs-4.18.0-193.6.3.el8_2.x86_64.img selinux=0 rd.shell rd.debug root=live:http://10.10.0.1/squashfs.img rw console=tty0 console=ttyS1,115200
initrd http://10.10.0.1/initramfs-4.18.0-193.6.3.el8_2.x86_64.img
boot
Which generate this in the console:
http://10.10.0.1/vmlinuz-4.18.0-193.6.3.el8_2.x86_64... ok
http://10.10.0.1/initramfs-4.18.0-193.6.3.el8_2.x86_64.img... ok
INTEL 0x6f080f70 MAC reset (081c0261/80280783 was 081c0261/80280783)
INTEL 0x6f080f70 MAC reset (081c0261/80280783 was 081c0261/80280783)
INTEL 0x6f081ab0 MAC reset (081c0261/80280787 was 081c0261/80280787)
[ 0.000000] Linux version 4.18.0-193.6.3.el8_2.x86_64 (mockbuild#kbuilder.bsys.centos.org) (gcc version 8.3.1 20191121 (Red Hat 8.3.1-5) (GCC)) #1 SMP Wed Jun 10 11:09:32 UTC 2020
[ 0.000000] Command line: vmlinuz-4.18.0-193.6.3.el8_2.x86_64 initrd=initramfs-4.18.0-193.6.3.el8_2.x86_64.img selinux=0 rd.shell rd.debug root=live:http://10.10.0.1/squashfs.img rw console=tty0 console=ttyS1,115200
...
On the server side, on apache logs I have:
10.10.2.1 - - [26/Jun/2020:19:55:42 +0200] "GET /vmlinuz-4.18.0-193.6.3.el8_2.x86_64 HTTP/1.1" 200 8913656 "-" "iPXE/1.0.0+"
10.10.2.1 - - [26/Jun/2020:19:55:42 +0200] "GET /initramfs-4.18.0-193.6.3.el8_2.x86_64.img HTTP/1.1" 200 53703611 "-" "iPXE/1.0.0+"
So it seems to be working perfectly.
This is diskless boot here, but whatever I try (kickstart diskfull install, or even kernel+initrd alone) I always end up to this kernel panic...
I tried to reset BIOS settings, try to boot in legacy/pcbios instead of UEFI, tried to desactivate sata disks, etc. Always this same error. Also tried to use kernel+initrd from Centos ISO (checked checksum), tried to use the ones from my management node. Nothing. Do I miss something obvious?
Does any of you have an idea or already faced this kind of issue?
Many thanks in advance :-)
With my best regards
Beuk