Shamim's Plugins

This should not be issue. Can you disable this option then try?

If still not working can you deactivate all plugins and change theme to one of default them then try? Then enable one by one and test?

If still not working please give me admin and ftp (only for this plugin access is ok) access also for that staging site using https://www.shamimsplugins.com/sensitive-information/

Please deactivate “Front End PM – Ultimate Member Integration” and activate again then test.
If still not working can you give me access to your website so that i can have a look? If yes, please create 2 test user account(1 as admin) and send using https://www.shamimsplugins.com/sensitive-information/

It depends on where you want to show that button.
Add following code in your theme’s (child theme’s if you are using) functions.php

add_action( 'fep_display_after_parent_message', function(){
	if ( fep_is_group_message( fep_get_the_id() ) ) {
		echo '<a href="' . fep_query_url( 'newmessage', array( 'fep_to' => fep_get_userdata( fep_get_message_field( 'mgs_author' ), 'user_nicename', 'id' ), 'message_title' => fep_get_the_title() ) ) . '" class="fep-button">Reply to Sender</a>';
	}
});

It will show a button to reply to sender bellow your main message content if that is a group message.

1. Please recheck if you set “Front End PM Page” in Front End PM PRO > Settings > General
2. If you deactivate that plugin, does it works?

Let me know.

Please go to Dashboard > Front End PM PRO > All Messages and select your messages and bulk delete.

Your code is correct except last semi-colon.

I see you are using a custom plugin for this plugin. In that plugin you are using get_current_screen() function without checking if that function exists or not.

Please go to custom-fepm-config.php around line 56 and add function_exists( 'get_current_screen' ) check before that.

How to add debug code?