I have a course section description that we are no longer able to edit. When opened, this is what the error is:

Even when we change the view in TinyMCE to HTML, the complete text-area is shown as empty.

I had to finally edit this through the database. Looking in the content of the section description, I noticed some weird HTML codes that probably break TinyMCE:

<b color: rgb(51, 51, 51);"><span lang="EN-US" class="" style="color: rgb(125, 159, 211);">

Further checking is needed to see if this code actually breaks TinyMCE.