Hello Christiaanb,
We apologize for the inconvenience caused, as per the description above, you are not able to combine PDF files as getting an error message, Is that correct?
Please check for any pending updates of Acrobat from help>check for updates, reboot the machine after installing the updates and see if this brings any difference.
You can also try repairing the Acrobat from help>repair installation.
Please refer and try the solution from the following Adobe article Troubleshoot Acrobat PDFMaker issues in Microsoft Office on Windows
If the issue persists, please share the following details:
- The dot version of Acrobat installed, to identify refer to Identify the product and its version for Acrobat and Reader DC
- The version of operating system installed?
- Is this error message comes while combining any PDF files, or specific to one or two PDF files?
- Make sure that the latest updates of the Operating system and Office is installed.
Let us know how it goes and share your findings.
Regards,
Anand Sri.