When we upgraded a client from 5.6.1 to 5.6.3, TinyMCE editors were stuck in **text** mode. Strangely, only the 2nd and on were affected. Clicking on the “visual” tab triggered an error from within core TinyMCE.
Another hint: problem went away when we disabled all other plugins AND switched to Twentyfifteen.
Also, we are not using Types Toolset.
