Admin: the developer was trying things, but without success. I just fooled around with it for a long time, and eventually figured a work around which seems to work. I loaded a new set of default buttons, (lost our previous set in the process) and then started the redo from there, and so far, no more problems.
The “Visual” buttons are arranged slightly differently….note especially that the grayed out “chain link” buttons are now in the middle of the 4th row, just before the image insert, and video insert, buttons.
As always, holding the cursor on the button will eventually bring up a descriptor for that button’s function. Let me know if this present TinyMCE doesn’t work for you.