Error:There was an error opening this document. The file is damaged...! on combining PDFs

About a week ago a user started getting reports that files she sent were getting the error "There was an error opening this document. The file is damaged and could not be repaired" when she creates combined pdfs in Acorbat Pro desktop.

I tried upating Acrobat, repairing the instalation, uninstalling and reinstalling the previous version. I tried disabling security mode on the users Adobe before creating the doucment. I tried disalabing security mode on my computer before opening. We tried viewing the document in the Chrome and Edge browser - no error but also no file contents showed on the screen. I tried clearing temporary files. I tried adding the bValidateBytesBeforeHeader key to the registry on both computers. Still getting the error. Finally I had the user try to combine the files using the online version of Acrobat and I was able to open the doucment without error, but the user needs to be able to combine the files in the desktop version of Acrobat Pro. 

What other steps can I try to fix this issue?