When you want to delete a document or folder you will see an error message informing you that links to this element exist that prevent its deletion.

You need to remove the links in connection with the file that you want to delete:

1. Links in other files that point to this file: in the details view under the tab Referenzen you will find a list of files that contain a links/links to the file to be deleted. If you click on the arrow-icon you will see a list of the names of all linked files and your file/document should be one of them. You will need to remove all the links pointing to the file that you wish to delete.
2. Links in the file to be deleted can be removed easily by breaking the link in the editor or else by removing the link from an input field in the properties section of the document or folder.