This signifies an issue with GFR Servers
Please notify them so they can resolve the issue : 800.968.0600 option 4
A temporary work around is:
make sure each document has a page count. The way to do this is to open the document for editing and then save the document back to GFR.
After doing this, refresh your documents to verify that they now have page counts – this sometimes takes a few moments. After these steps the documents will merge without errors.