How to downgrade:
DisableCollabora Online App in Apps section
Remove Collabora Online App
Connect to your Nextcloud instance via SSH and go into the nextcloud folder/docker volume of your Nextcloud Instance and cd into the apps folder. There you can download the old version
https://github.com/nextcloud/richdocuments/releases/download/v3.5.3/richdocuments.tar.gz 86» and unpack it in the apps folder.
Go back in the Apps Section of your Nextcloud instance and enable the Collabora Online App again and try editing a document