Error combining PDF files in Adobe Acrobat

You'll get the error

 

This is a known issue if you have Acrobat and Reader installed on the same server:
https://helpx.adobe.com/acrobat/kb/known-issues-acrobat-x-reader.html

 

• You can't combine files if you have both Acrobat and Reader installed and also have a PDF open in Reader (or Reader running in the background). If you right-click a file of a supported format and choose Combine Supported Files In Acrobat, the Combine Files dialog box doesn't open. The same result occurs if you choose Run and type acrobat /n. The workaround is to use the Run command to start Acrobat. Then click the Create button in the toolbar and choose Combine Files Into A Single PDF. [2605772]  

Suggested workaround is to add Adobe Acrobat to the Startup folder, with the '/h' parameter specified (start in minimized mode). This way, Acrobat will automatically start up and most of the time you won't notice its running.