ServiceTag should never be empty. Couple of things to clarify:
Port 443 is used for discovery of iDRAC and ESXi servers using WSMAN protocol.
For SNMP, port 161 should be opened on the managed servers (with OMSA installed) for OME to discover these servers.
WIndows 2003 may not be officially supported in later versions of OMSA 7.x.
Are the servers being discovered using SNMP protocol ?
Does all these servers have OMSA installed ?
Was motherboards replaced on these servers ?
Recommended to discover iDRAC 6 and 7 using WSMAN protocol and secure mode enabled in OME discovery wizard.
Thanks,
Raj Shresta