Combining files no longer works
I used to be able to select "File / Create / Combine Files Into A Single PDF" and combine files into a single PDF.
This no longer works.
It now generates a new left-hand window that shows multiple PDFs instead of the pages of the PDF. I can no longer view the pages of the combined PDF but only the two files, stacked "on top of" each other.

I can no longer save it as a standard PDF but now only as a "Portfolio" PDF.
Upon saving, it no longer can be opened in Acrobat Pro DC and now has an error screen telling me to open it in Acrobat X.
How can I restore the ability to combine files into a PDF?
