We need a lot more information to be able to help you:
Where is your nextcloud running? (VPS/Raspy/Webspace)
What OS / way auf setup?
What does the OnlyOffice settings page show you?
Is there anything in the logs? (apache? nextcloud?)
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Why did you change the serveradress of OnlyOffice? Should not be “rainer” if you want to use the community document server…
The community document server will automatically configure itself if no other document server is configured in the ONLYOFFICE settings (“Document Editing Service address” is empty). All other “Server settings” should be left empty.