Unread messages remain unread

Are you using any caching plugin?
You have written in Chrome browser, What about other browser?

I’m not using any caching plugin. The same applies on Firefox.
It is easy to reproduce.
We have 2 people “A” and “B”. “A” sends a message to “B”. “B” reads the message and returns to inbox. Everything is fine the status of the message in the inbox is updated from “unread” to “read”. Then “B” sends a reply to “A” opening the message that “A” sent to “B” and by replying on the same thread. “A” receives the messages, reads the message (does not reply) and returns to inbox. Status of the message remains “unread”. What do you think?

Thank you for explain nicely.

Please open fep-class.php and find in around line 423

if ($post->status == 0 && $user_ID == $post->to_user) //Update only if the reader is the reciever
	  	$wpdb->update( FEP_MESSAGES_TABLE, array( 'status' => 1 ), array( 'id' => $pID ), array( '%d' ), array( '%d' ));

Replace with

if ($post->status == 0 && $user_ID != $post->last_sender && ( $user_ID == $post->from_user || $user_ID == $post->to_user )) //Update only if the reader is not last sender
	  	$wpdb->update( FEP_MESSAGES_TABLE, array( 'status' => 1 ), array( 'id' => $post->id ), array( '%d' ), array( '%d' ));

Hope this will solve this bug. I have also updated plugin’s file in wordpress. So you can also download again from wordpress.

Well done thank you.