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:

• It needs that ominous “community document server”. It is best to install this first and make sure that the server environment provides a php memory_limit of at least 512mb. Change in the global php_ini if necessary. To install, simply search for “Community Document Server” in the upper right corner.
• Only then install the actual OnlyOffice via the NextCloud app installer.
• If something does not fit then simply delete all entries from OnlyOffice (“oc_appconfig”)in the NextCloud SQL database.
• Then another small “failure”: the Office package is not found as such, but only as a “file type”, that is simply go to FILES and there you can then create your own Office Doc on your own cloud. Yeah!

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

https://help.nextcloud.com/t/falscher-privater-schlussel-fur-die-verschlusselungs-app-bitte-aktualisieren-sie-ihren-privaten-schlussel-in-ihren-personlichen-einstellungen-um-wieder-zugriff-auf-die-verschlusselten-dateien-zu-erhalten/22659/4

or WTF! https://justlinuxblog.wordpress.com/2019/01/20/nextcloud-fehler-falscher-privater-schluessel/