Strange connection issue manifested after server location move.
Database: XE18 running on windows 2019 standard edition
I recently upgraded to XE18 from XE11, with client apps connecting to it via 12c client, there were no issues once I did the initial upgrade and then imported needed schemas only after a few practice runs. There is only one PDB called XEPDB1. The NIC of the server has 3 ports, one for maintenance, one with a fixed IP and one with DHCP, only the maintenance & fixed IP ports being used.
When the tech powered down & moved the server, he at first plugged both network cables into the fixed IP and DHCP ports, then realized what he did, changed to the way it was before and restarted server.