403 Forbidden error / page=functions.php (port 80)

I keep getting this error when I try to save the customization settings of the theme. i.e the custom settings that many themes have.

For example the theme has links to various social profiles. When I update the links and hit save I get this error. Ditto when adding any google analytics code.

Any idea why this error comes up?

403 Forbidden
Access is forbidden to the requested page:

/wp-admin/admin.php?page=functions.php (port 80)
Please forward this error screen to ***.com’s WebMaster.