When trying to upload the book, the system left the space for author empty. And it was not possible to write anything to the space. The book has a front page with editors clearly displayed.
For books, RG provides two fields for names: "Authors" and "Editor". However, the editor field cannot be connected to profiles, only the authors' names are connected to their profiles. Therefore, you may think about adding the names of the editors also to the Authors field (like I did in https://www.researchgate.net/publication/354678126). See "How do I edit my research item's details?" in https://explore.researchgate.net/display/support/Reviewing%2C+featuring%2C+and+editing+your+research for instructions how to change the various fields for an existing entry.
Thank you Wolfgang for your answer. Now I succeeded to upload the book with all needed metadata.