I'm getting exactly the same error on some PDF's. I suspect it's an error with fonts not matching up somehow since the font box comes up empty in the Tools/Content Editing window, but disappears when you click OK on the error window.
OK, called into Adobe tech support. Found out the error was in the PDF delivered from the website. It viewed fine, but in editing had problems. Solution was to print the original flawed PDF, to the Adobe PDF printer, and then edit the new file. This worked fine.
In cases like this it is always good to go to the document properties (ctrl-D) and check the generation process. That would have been a clue in your case. Glad you found a solution.