Copy/pasting from https://www.mediawiki.org/w/index.php?title=Topic:Tnn0sniwn09vff41&action=history :

Problem

When attempting to see changes in a page the editor shows the error "Error loading data from server: apierror-visualeditor-docserver-http."

Steps to reproduce:

Go to VisualEditor
Click edit to trigger the wikitext editor (alt + shift + e)
Click the pencil icon, and click the button to switch to VisualEditor
Add some text before the first line , e.g. "bad bug"
Click Publish changes
Click Visual
Expected

Changes shown visibly in the document.

Output:

A dialog with the message "Error loading data from server: apierror-visualeditor-docserver-http."

Workaround:

Cancel the edit, trigger VisualEditor directly using (alt + shift + v) .

Console error:

Uncaught TypeError: Cannot read property 'diff' of undefinedVeUiDiffElement @ VM955:formatted:10240ve.ui.MWSaveDialog.updateReviewMode @ VM955:formatted:18452oo.EventEmitter.emit @ VM952:7OO.ui.SelectWidget.selectItem @ load.php?debug=false&lang=en&modules=oojs-ui-core|oojs-ui.styles.icons%2Cicons-editing-advanced&ski…:81OO.ui.SelectWidget.chooseItem @ load.php?debug=false&lang=en&modules=oojs-ui-core|oojs-ui.styles.icons%2Cicons-editing-advanced&ski…:82OO.ui.SelectWidget.onMouseUp @ load.php?debug=false&lang=en&modules=oojs-ui-core|oojs-ui.styles.icons%2Cicons-editing-advanced&ski…:75
Line:VM955:formatted:10240

Content:

var diff = visualDiff.diff;