Some sequence of enabling the footer with widgets, and deleting all widgets from the footer in Appearance -> Widgets seems to have resolved the issue.
I am now able to display the footer without error.
I’m seeing the uncaught exception below even with SG Optimizer disabled on the subdomain:
Uncaught SyntaxError: Unexpected string avia-footer-scripts-…-659ff2ec799fc.js:7
I also note that disabling display of the socket and footer widgets completely does resolve the issue.