Advanced Custom Fields - Fatal error: Uncaught TypeError: Illegal offset type in isset or empty in...

Overview of the issue

The following fatal error may occur when using Advanced Custom Fields plugin. The error is specifically observed with PHP 8 or higher:

Fatal error: Uncaught TypeError: Illegal offset type in isset or empty in .../wp-content/plugins/sitepress-multilingual-cms/classes/core-abstract-classes/class-wpml-element-translation.php:259

Workaround

Please, make sure of having a full backup of your site before proceeding.

• Open …/wp-content/plugins/acfml/classes/class-wpml-acf-attachments.php file.
• Look for line 28.
• Replace:

  			foreach ( $attachments as $key => $attachment_id ) {
  					$translated_attachments[ $key ] = apply_filters( 'wpml_object_id', $attachment_id, 'attachment', true );
  

• With:

  			foreach ( $attachments as $key => $attachment_id ) {
  				if (is_int($attachment_id)) {
  					$translated_attachments[ $key ] = apply_filters( 'wpml_object_id', $attachment_id, 'attachment', true );
  				}