LEAP 15.5 (after upgrade) will not boot with GRUB2-EFI

Following upgrade to LEAP 15.5 from 15.4 server will no longer boot automatically. What happens is that “Welcome to GRUB” briefly flashes on the screen and then the server reboots. I can boot using an EFI Rescue USB disk and entering the root=, linux, initrd and boot commands in GRUB.

In BIOS I have set UEFI Only and Secure Boot off. I have verified with efibootmgr that all the drives are correct. I have used yast2 and also have rebuilt /boot/grub2/grub.cfg by hand after verifying all the parameterization files. I have read extensively on the internet and inspected all of the files and process. I cannot see what I am missing, but I am obviously missing something,

Machine:
Custom built tower server
CPU: 8x Intel(R) Core™ i7-4770 CPU @ 3.40GHz
BIOS: American Megatrends Inc. version F6

Linux:

server:~ # uname -a
Linux server 5.14.21-150500.55.31-default #1 SMP PREEMPT_DYNAMIC Wed Oct 4 16:52:05 UTC 2023 (5dc23e0) x86_64 x86_64 x86_64 GNU/Linux

efibootmgr:

server:~ # efibootmgr -v
BootCurrent: 0006
Timeout: 1 seconds
BootOrder: 0000,0006,0007
Boot0000* opensuse      HD(1,GPT,9882d538-2462-4502-994f-05c6845f5d0a,0x800,0x3ff000)/File(\EFI\opensuse\grubx64.efi)
Boot0006* UEFI: SanDisk HD(1,MBR,0x24a8c920,0xf4,0xb40)/File(\EFI\BOOT\BOOTX64.EFI)AMBO
Boot0007* UEFI OS       HD(1,GPT,9882d538-2462-4502-994f-05c6845f5d0a,0x800,0x3ff000)/File(\EFI\BOOT\BOOTX64.EFI)

/etc/fstab:

server:~ # cat /etc/fstab
/dev/system/swap     swap                 swap       defaults              0 0
/dev/system/root     /                    ext4       acl,user_xattr        1 1
/dev/disk/by-id/ata-ST500DM002-1BD142_W3T0H2JS-part1 /boot/efi            vfat       umask=0002,utf8=true  0 0

fdisk:

server:~ # fdisk -l
Disk /dev/sda: 465.76 GiB, 500107862016 bytes, 976773168 sectors
Disk model: ST500DM002-1BD14
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: F95C856A-0B70-4F00-A468-11260BDC6E63

Device       Start       End   Sectors   Size Type
/dev/sda1     2048   4192255   4190208     2G EFI System
/dev/sda2  4192256 976773119 972580864 463.8G Linux RAID

Disk /dev/sdb: 931.51 GiB, 1000204886016 bytes, 1953525168 sectors
Disk model: WDC WD10EZEX-08W
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0x9b633292

Device     Boot   Start       End   Sectors   Size Id Type
/dev/sdb1  *       2048   4192255   4190208     2G  6 FAT16
/dev/sdb2       4192256 976773119 972580864 463.8G fd Linux raid autodetect

Disk /dev/md0: 463.76 GiB, 497961205760 bytes, 972580480 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Disk /dev/mapper/system-swap: 8 GiB, 8589934592 bytes, 16777216 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Disk /dev/mapper/system-root: 455.75 GiB, 489362030592 bytes, 955785216 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

/etc/default/grub:

server:~ # cat /etc/default/grub
GRUB_DISABLE_OS_PROBER="true"
GRUB_TERMINAL="gfxterm"
GRUB_HIDDEN_TIMEOUT="0"
GRUB_TIMEOUT="8"
GRUB_ENABLE_CRYPTODISK="n"
GRUB_GFXMODE="auto"
GRUB_DISABLE_RECOVERY="true"
GRUB_DISTRIBUTOR=
GRUB_DEFAULT="saved"
SUSE_BTRFS_SNAPSHOT_BOOTING="true"
GRUB_USE_LINUXEFI="true"
GRUB_CMDLINE_LINUX_DEFAULT="quiet mitigations=auto"
GRUB_CMDLINE_XEN_DEFAULT="vga=gfx-1024x768x16"

grub2-mkconfig:

server:~ # grub2-mkconfig -o /boot/grub2/grub.cfg
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.14.21-150500.55.31-default
Found initrd image: /boot/initrd-5.14.21-150500.55.31-default
Found linux image: /boot/vmlinuz-5.14.21-150500.55.31-debug
Found initrd image: /boot/initrd-5.14.21-150500.55.31-debug
Found linux image: /boot/vmlinuz-5.14.21-150400.24.88-default
Found initrd image: /boot/initrd-5.14.21-150400.24.88-default
Found linux image: /boot/vmlinuz-5.14.21-150400.24.88-debug
Found initrd image: /boot/initrd-5.14.21-150400.24.88-debug
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
Adding boot menu entry for UEFI Firmware Settings ...
done

I have rebuilt /boot/grub2/grub.cfg by hand and with yast2. Same results both ways. Whichever boot option I force the BIOS to select (starting with opensuse) the message “Welcome to GRUB” flashes briefly and then the computer immediately reboots.

My hypothesis is that the problem is not with the BIOS, but with GRUB2 given the boot process gets as far as flashing the message “Welcome to GRUB” - AND - I can boot with the EFI Rescue USB disk.

If anyone can spot something I missed or give me a clue how to investigate further, I would be grateful.

Thanks,

Gato