Visual Editor in the PODs fields is not working
Does the site have PODs?
Does the POD have a WSYWIG field?
Does only the HTML editor work and not the visual editor?
If you come across this, the fix appears to be quite simple.
In the PODs settings, click on the Rest API tab for the POD in question and make sure the Enabled box is NOT checked.
NOTE: This is for the POD itself, not the individual fields.
Flush all the Caches and now your visual editor should work.