In order to get new messages the chat is requesting the following URL:
https://yourdomain/wp-content/plugins/wise-chat-pro/src/endpoints
It is very likely the URL is is forbidden in your setup (403 error). It may be caused by a security plugin or theme. A specific configuration of HTTP server (.htaccess file or server settings) may be the reason too. Please ask your server administrator to make the URL accessible.
Please read more about it here.