Document server connection OnlyOffice 403 forbidden

Hi, it would be great to edit office files online.
Thus, I installed the OnlyOffice Connector and the Community Document Server.
However, when saving the OnlyOffice setting with the supposed document server directory, I get the following notice:
Error when trying to connect (Client error: POST resulted in a 403 Forbidden response: <!DOCTYPE html> <html> <head> <meta charset="utf-8"> <style type="text/css"> html, body, (truncated...) )
Any idea?
Nextcloud is 18.06 and PHP 7.4.7
Best regards,