The goal here is to be able to modify the default gateway IP address on the Management Center. An attempt was aborted, with the "application protocol error" > "Error: failed to apply modifications". Se full error in the snippet added in the resolution section.
An attempt at making this modification from the SSH access and received the "Failed to applied modification" error. See snippet below, for the full error.
Apparently, this change, being a setup config, done with the highest local privilege on the appliance, should happen from the serial console access, by going through the same "Initial Configuration" steps but only modifying the IP address of the default gateway, to the desired IP address. Guidance was provided through the steps and the default gateway IP address modification was done successfully.
Note: Ensure the new gateway IP address is reachable from the MC appliance, before making the change. This connectivity was validated, in this case.
This resolution was applied to case 32906265, successfully.