Follow up. I believe I found the source of the invalid line of code you found in the wpconfig.php file. To assist him in migrating my site, my site developer used a plugin called Root Relative URLs. That plugin’s installation instructions https://wordpress.org/plugins/root-relative-urls/installation/ say to put the line you found (among others) in the wpconfig.php file. He left those lines in after migration, but we know now that he should have removed them.
Thanks again for all your help and a fabulous plugin.
Jeff