How can solve Warning: Cannot modify header information – headers already sent by (output started at wp-includes/functions.php:5275) in wp-content/plugins/user-submitted-posts/user-submitted-posts.php on line 1622

An error like this can be caused by some code in the theme or a plugin that tries to send headers after the page content has started outputtng.

Could you try to edit your theme functions.php and add this (inside the tags):

add_filter('template_redirect', function () {
  ob_start(null, 0, 0);
});

Are you still experiencing the same problem after that change?