Customizer admin-ajax.php 404

When attempting to save changes in the customizer(/wp-admin/customize.php) I’m getting a 404 error from admin-ajax.php. I get a 200 from admin-ajax.php when modifying widgets (/wp-admin/widgets.php).

Any ideas what’s different in the customizer context that would result in a different response behavior?

In searching, people have asked to verify
1: that the file exists: yes it does
2: permissions: 640, which is correct for my environment. I changed to 777 to test but it made no difference.

Thanks for any ideas