Please check the value of Editor.EditorWysiwygModeCss Property. If the above style sheet doesn't exist, please make sure this property doesn't call it.
Editor.EditorWysiwygModeCss Property
Specifies the location of the style sheet that will be used by the editable area. Multiple Style Sheets are supported. Example EditorWysiwygModeCss="example.css,~/portal.css,/default.css"
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