Sorry, this problem seems difficult to solve...

[    0.428620] APIC: NR_CPUS/possible_cpus limit of 256 reached. Processor 378/0x195 ignored.
[    0.428621] APIC: NR_CPUS/possible_cpus limit of 256 reached. Processor 379/0x197 ignored.
[    0.428621] APIC: NR_CPUS/possible_cpus limit of 256 reached. Processor 380/0x199 ignored.
[    0.428622] APIC: NR_CPUS/possible_cpus limit of 256 reached. Processor 381/0x19b ignored.
[    0.428622] APIC: NR_CPUS/possible_cpus limit of 256 reached. Processor 382/0x19d ignored.
[    0.428623] APIC: NR_CPUS/possible_cpus limit of 256 reached. Processor 383/0x19f ignored.
[    0.428706] smpboot: 384 Processors exceeds NR_CPUS limit of 256
[    0.428707] smpboot: Allowing 256 CPUs, 0 hotplug CPUs
[    0.432561] setup_percpu: NR_CPUS:256 nr_cpumask_bits:256 nr_cpu_ids:256 nr_node_ids:24
[    1.015021] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=256, Nodes=24

apic error are nown acpi windows motherboard BS... there is no solve until we hold Motherboard manufactures responsible for proper acpi implementations... it is safe to ignore.

extra sylinx/grub boot option such as noapic may help here...
 

 

The issue here seems to stem from the limitation on the number of CPUs (NR_CPUS) that the kernel can handle, with the motherboard's BIOS/UEFI ACPI table possibly contributing to the problem. The kernel message indicates that it's detecting more CPUs than the configured NR_CPUS limit (256), and this can sometimes happen when the BIOS/UEFI provides inaccurate information about hardware configurations.

Here are some potential solutions to try:
 

Check for BIOS/UEFI Updates:

Updating the BIOS/UEFI firmware might resolve the problem if the motherboard manufacturer has provided a fix. Since this motherboard manufacturer (XYZ) uses similar BIOS/UEFI images across models, an update could correct the ACPI table entries.

Limit CPU Count via Kernel Parameters:

You can set a limit on the number of CPUs recognized by adding a kernel boot parameter, such as maxcpus=256 or nr_cpus=256, to restrict the kernel from trying to detect more than 256 CPUs.
(Mostlikly would needs windos to flash... maybe lucky to be able to downlad to unraid flash and use bios to read write update...)
 

To do this:

Edit the boot configuration file (e.g., /boot/syslinux/syslinux.cfg or /boot/grub/grub.cfg depending on the bootloader).
Append maxcpus=256 to the kernel parameters.

clic main > flash> and add 
 

maxcpus=256 nr_cpus=256

to syslinux...
 

Check ACPI Tables:

You could use tools like dmidecode or acpidump to analyze the ACPI tables and identify mismatches. If there's an option to override specific entries (some advanced setups allow ACPI table patching), this might help align the ACPI entries with the actual hardware.

If none of these work and the system doesn't require all cores, limiting the CPU count is usually the simplest and most stable workaround.

but its a konwn issues with motherboards due to improper acpi windows implementations...