Acrobat Online- Combine 8 PDFs into 1 PDF - undefined error

[L5CCMJK8]

Trying to merge 8 PDF files into 1 PDF.

The message in Acrobat (Web) is very cryptic "Error in Merging" "Files cannot be extracted" (in German, attached). Maybe Ok. But 1)  all these files can be opened in Adobe Acrobat. 2) There is no indication on which file is failing => How can I get this information or more information to correct the issue?

[try67]

I agree it's not a very good error message, but the fact you can open the files in Acrobat doesn't mean they can be merged.

Check if any of them is secured (File - Properties - Security), digitally signed (Signatures panel) or was created using LiveCycle Designer (File - Properties - Description). If that's the case, that would explain it.