I just tested this on a site with only ACF and WP, I tested with both ACF4 and ACF5. I’m not getting this error. Have you tried deactivating plugins or changing themes?
Are the WYSIWYG fields at the top level or are they in a repeater or something? What are the settings for the fields. If you give me some more information I’ll try more testing if I am able to. I don’t have the flex field for ACF 4 though so I can only go so far with that.
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