Message Box Problem

I’m having similar problems to a recent poster. I have the latest WP (3.9.1) and latest email users installed. If I attempt a mailing, I can select a group (or an individual) and I can also type in the subject box. However, if the message box is set to ‘text’ I can type into it. But if I switch to Visual mode, my text disappears and I am unable to type anything further in the message box. Nor can I switch back to text mode. If I try, all I can see is the formatting bar of the message box, but no actual box in which to type. This was discovered about two weeks ago by another user, but only communicated to me yesterday. Using Firebug, I get this error message:

ReferenceError: downloadsEdL10n is not defined
(function(){tinymce.PluginManager.add("downloadmanager",function(a,c){a.addCommand("WP-DownloadManager-Insert_Download",function(){var b=jQuery.trim(prompt(downloadsEdL10n.enter_download_id));null!=b&&""!=b&&a.insertContent('[download="'+b+'"]')});a.addButton("downloadmanager",{text:!1,tooltip:downloadsEdL10n.insert_download,icon:"downloadmanager dashicons-before dashicons-download",onclick:function(){tinyMCE.activeEditor.execCommand("WP-DownloadManager-Insert_Download")}})})})();

I am not competent to understand this! Any help appreciated.

https://www.ads-software.com/plugins/email-users/