No connection to OnlyOffice

Hey,

set up fresh Nextcloud of days ago.

Running Nextcloud 31.0.4 via Container on Synology (720+, DSM 7.2.2).
Running Onlyoffice via Container on same Synology.

Created container Onlyoffice with

docker run -d --name=OnlyOffice
-p 8467:80
-v /volume1/docker/onlyoffice/logs:/var/log/onlyoffice
-v /volume1/docker/onlyoffice/data:/var/www/onlyoffice/Data
-e JWT_ENABLED=true
-e JWT_SECRET=(mysecret)
–restart always
onlyoffice/documentserver

Both applications work. Access to Onlyoffice via https://onlyoffice.(myid).synology.me/, welcome page loads.

Installed the Onlyoffice-app in Nextcloud, configured settings as follows:

As soon as I click “Save” following error is produced:

What’s going on, what can I do?

Is there anything I need to modify in the penultimate field where the standard entry says https : // < documentserver >/ ?