I found this: https://support.microsoft.com/en-us/topic/considerations-for-server-side-automation-of-office-48bcfe93-8a89-47f1-0bce-017433ad79e2

Microsoft does not recommend or support server-side Automation of Office

I have given up trying with MS Office and am now using LibreOffice. This worked immediately and without any problems. I can't see any difference in the quality of our documents compared to documents exported with MS Office.

soffice --headless --convert-to pdf --outdir $ExportFolder $DocPath