WordPress converts "&" to "&" when the hidden field used to store a return URL is saved.  This results in being redirected to an invalid URL after an entry is updated.  The following code added to your child theme functions.php solves the problem discussed here: https://Formidableforms.com/help-desk/using-return-to-previous-page-after-edit-code-has-issues-with/.  As always, make sure you have FTP access to your site before editing functions.php from the WordPress backend in case something goes wrong.

add_filter('frmpro_fields_replace_shortcodes', 'frm_replace_amp_entity_with_amp', 10, 4);
function frm_replace_amp_entity_with_amp($value, $tag, $atts, $field){
 if ( isset($atts['amp']) ) {
 $value = str_replace( "&", "&", $value);
 }
 return $value;
}