Thread:Project:Support desk/fatal error message: FCKeditor/reply (9)

The other IP is right. However, take note that Extension:WYSIWYG has last been officially updated for MediaWiki 1.17. That is several years in the past. During that time, no updates have been done. That does not only mean no more bug fixes and features, but even more important: No security fixes. I would not consider a 3-year-unmaintained extension secure.