On on fresh installed wiki (1.26.2) the button 'Preview' and 'Changes' on top don't work.
Only a spinning wheel appears, but no content.
Tested with FF, chrome and IE

PS: old installation with wiki 1.23.6 and wikieditor 0.4.0 works well

System:
Product Version
MediaWiki 1.26.2
PHP 5.6.17-0+deb8u1 (apache2handler)
MySQL 5.5.47-0+deb8u1

Entry point URL
Article path /w/$1
Script path /w
index.php /w/index.php
api.php /w/api.php
load.php /w/load.php

Localsettings.php:
wfLoadExtension( 'WikiEditor' );

#Enables use of WikiEditor by default but still allow users to disable it in preferences
$wgDefaultUserOptions['usebetatoolbar'] = 1;
$wgDefaultUserOptions['usebetatoolbar-cgd'] = 1;

#Displays the Preview and Changes tabs
$wgDefaultUserOptions['wikieditor-preview'] = 1;

#Displays the Publish and Cancel buttons on the top right side
$wgDefaultUserOptions['wikieditor-publish'] = 0;