SWAG Nextcloud Upload not working


I have tried this on QNAP and Debian with the same result so it should be easy to reproduce:

  1. Fresh Install of SWAG and Nextcloud, nothing special.
  2. Configure Nextcloud Subdomain
  3. Create an empty ad.xml file and try to upload
  4. Receive an Unknown Error, no file uploaded and no clues in logs

Other .xml files are uploaded without problems but error with ad.xml.
Any Ideas why?