A recent update to the PDFLib tool has enforced the lack of support for setting both Trimboxes and Artboxes in PDF/X files. Only one can be set, not both. This will cause an error, and Advanced ezMerge will not generate a file.
To fix this, you will need to remove the PDF/X information from the file and reupload it into the Advanced ezMerge template.
Open your PDF in Adobe Acrobat Pro, go to View->Tools->Print Production and select Preflight.
Once the Preflight tool is open scroll to the PDF/X section, open it and select Remove PDF/X Information. Then click on Analyze and Fix. You will be asked to save the new file. Save it and then reupload it into the Advanced ezMerge template. This will fix the error message you are receiving.