Thank you all so much for your suggestions. I never figured out exactly what the problem was but after 48 hours, I used this technique:
So, when you get the over set text error message but can’t find anything visible (or invisible) for why it is happening, you can just delete that section of text–a little or a lot, or all the way to the end–by doing this:
Using the Type Tool, place the cursor at the end of the section of text you want to keep. Then use this shortcut:
Cmd/Control + Shift + End (or hold down the down arrow for just a sec) Then hit Delete to remove the text permanently. In this way I was able to delete a few paragraphs of a very long string of text (100 pp.) . Then I manually typed in the missing text. Sweet relief!