I found a solution, for those interested:
<?php
/*
* Plugin Name: URL fix
*/
/** PUT SOME CODE HERE **/
add_filter( 'caldera_forms_submission_url', function( $url, $form_id ){
$parsed=parse_url($url);
$url = "$parsed[scheme]://$parsed[host]/index.php$parsed[path]";
return $url;
});
?>
Save this as a .php, put it in wp-content\plugins folder and activate the plugin.
