Data transfer has failed

If you get the following error message at the top of WebMail Lite screen:

Data transfer has failed, probably due to server error. Please contact system administrator.

that usually means some PHP error has occured, and the error message broke the normal data exchange between web browser and web server.

It's recommended to check web server / PHP error logs to find out what's causing this. However, there's another way which might be much simpler for users: you'll need to open web browser console. In Google Chrome and similar browsers it's activated with F12 key, while in Mozilla Firefox it's Ctrl-Shift-K or similar key shortcut. If you install Firebug plugin for Mozilla Firefox, it's opened with F12 key as well.

Once the browser console is active, repeat the action which has triggered the error, and see what is the message recorded in the console. This information might be sufficient for eliminating the core reason of the issue.