Windows 10 1803 Requires SMB2 or Higher

From http://linux-ip.net/html/ether-arp.html:

If no ARP cache entry exists for a requested destination IP, the kernel will generate mcast_solicit ARP requests until receiving an answer. During this discovery period, the ARP cache entry will be listed in an incomplete state. If the lookup does not succeed after the specified number of ARP requests, the ARP cache entry will be listed in a failed state. If the lookup does succeed, the kernel enters the response into the ARP cache and resets the confirmation and update timers.

It looks like your gateway box is not responding (or responding too slowly) to ARP requests from your gateway box. Does that <incomplete> eventually switch to <failed>? What network hardware do you have between the the server and the gateway? Is it possible broadcast ARP requests are being filtered or blocked somewhere between the two hosts?

Active Directory Schema Version 44 is Windows Server 2008 RTM.

If you are going to dcpromo promote a Windows Server 2008 R2 member server to a domain controller, the schema version has to be at 47 -- Windows Server 2008 R2 RTM.

Run adprep from the \support\adprep frolder on the Windows Server 2008 R2 DVD on your Windows Server 2003 domain controller with the Schema Master FSMO role.

The included version of adprep.exe on the Windows Server 2008 R2 DVD is a 64 bit executable. If your Schema Master domain controller is a 32 bit version of Windows Server, use adprep32.exe instead.