This is happening to me as well. The same pink error box appears. Worse, when it was worst, no fields that used TinyMCE were editable. They just appeared blank.
After downgrading to the generic 2017 theme with no plugins, reactivating my theme (Suffusion) and then slowly building back up, I have it to where the error appears, but I can still edit text. I had to disable the following plugins to get editing functionality back:
?? UsageDD
Displays usage information to administrators
Version 1.4.2 | By Dion Designs | View details
Clean Options
finds orphaned options and allows for their removal from the wp_options table
Version 1.3.2 | By Mittineague | View details
Plugins Garbage Collector
It scans your WordPress database and shows what various things old plugins which were deactivated, uninstalled) left in it. The list of additional database tables used by plugins with quant of records, size, and plugin name is shown.
Version 0.10.3 | By Vladimir Garagulya | View details
Unfortunately, I don’t know which plugin causes the tinymce error to appear in the first place.