I received this error when only trying to combine/ insert a few pages. 1/5 of the pages had successfully combined with the primary page. I realized that I had saved this differently than the other from the Property Mgmt System we use; rather than use the Save icon, I had used the Print>Save as PDF (selected as printer) method. I did this for the other pages and was able to successfully avoid further error messages. I did note, however, that if I tried to replace the current files (rather than print/save as a new file) it gave me the error again. Cheers.