There were some problems and did not want to run with me but to make it short, here all the little problems I had on the way to the DSGVO-compliant, own “Google Doc Clone” documented:
Another thing is just such a little old NextCloud bug that is not directly related to OnlyOffice but produces this error message:
Incorrect private key for the encryption app. Please update your private key in your personal settings to get access to the encrypted files again.
The solution to that is… Default encryption module
or WTF! https://justlinuxblog.wordpress.com/2019/01/20/nextcloud-fehler-falscher-privater-schluessel/