It turned out to be a problem with the uploads directory (not sure what caused it initially). I was able to resolve the issue with the following filter:
function fix_ssl( $url ) {
if ( is_ssl() ) {
if (stripos($url, 'http://') === 0) {
$url="https" . substr($url, 4);
}
}
return $url;
}
function uploadDir($uploads) {
$uploads['url'] = fix_ssl($uploads['url']);
$uploads['baseurl'] = fix_ssl($uploads['baseurl']);
return $uploads;
}
add_filter('upload_dir', 'uploadDir');