When using IIIF manifests generated by the Omeka S IIIF Server module with the Image Server disabled (described below), image loading errors due to CORS policy were observed, as shown in the figure below. These messages can be confirmed in each browser's developer tools.
This issue may occur when the "Default IIIF image api version" is set to "No image server" in the IIIF Server module settings, as shown in the figure below.
As a workaround, this can be resolved by placing a file named ".htaccess" with the following content directly under "(Omeka S installation directory)/files".
Header set Access-Control-Allow-Origin "*"