We are sorry for the trouble. As described you are getting the unidentified error when combining the PDFs
Would you mind sharing the version of the Adobe Acrobat DC you are using? To check the version of the application please go to Help > About Adobe Acrobat DC and make sure you have the latest version 20.12.20041 installed. Go to Help > Check for Updates.
Also, please try with a different set of PDF file and try to combine and see if that goes well.