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Solved!
On the next website I found the answer. https://www.agentwp.com/fix-thumbnails-not-working-in-a-wordpress-theme-from-elegant-themes
The parts that worked voor me were:
If thumbnails are still not working on your website, then login via FTP to your web server and make sure that the uploads and the et_temp folders are fully writable, that is, they have 755 or 777 permission depending upon the security configuration of your web server. Try 755 first as its more secure.
and:
Appearance > Editor in your WordPress dashboard, and select custom-functions.php file from right side to edit.
Find the following code in this file, and comment the marked lines,
if ( false !== get_option( 'et_images_temp_folder' ) ) return;
$uploads_dir = wp_upload_dir();
$destination_dir = ( false === $uploads_dir['error'] ) ? path_join( $uploads_dir['basedir'], 'et_temp' ) : null;
if ( ! wp_mkdir_p( $destination_dir ) ) update_option( 'et_images_temp_folder', '' );
else {
update_option( 'et_images_temp_folder', preg_replace( '#\/\/#', '/', $destination_dir ) );
update_option( 'et_schedule_clean_images_last_time', time() );
}
The edited code snippet will look like this,
#if ( false !== get_option( 'et_images_temp_folder' ) ) return;
$uploads_dir = wp_upload_dir();
$destination_dir = ( false === $uploads_dir['error'] ) ? path_join( $uploads_dir['basedir'], 'et_temp' ) : null;
#if ( ! wp_mkdir_p( $destination_dir ) ) update_option( 'et_images_temp_folder', '' );
#else {
update_option( 'et_images_temp_folder', preg_replace( '#\/\/#', '/', $destination_dir ) );
update_option( 'et_schedule_clean_images_last_time', time() );
#}
Now save this file and go to your homepage and reload the page. The thumbnails will now appear fine.
And it works!