It's hard to say what could be causing it, but sometimes this error is also telling you there was an issue with the UI either with a default or custom widget. It would be ideal if you could reproduce this locally but you can ask the user to open the Browser Console (Usually using the F12 key) when it happens and capture any logs found there e.g. Chrome (https://developers.google.com/web/tools/chrome-devtools/console/):
We get this error from time to time. It is nothing more than a temporary network/server issue. It was annoying for users so we decided to hide the popup with the following javascript
Use this javascript to hide the connection error popup
if ( window.connectionErrorOveride != true ) {
window.connectionErrorOveride = true;
var oldTranslate = mx.ui.translate ;
mx.ui.translate = function(t, e, n, i) {
if ( e == "connection_error" ) {
return null;
} else {
return oldTranslate.apply(this, arguments);
}
}
}