Just for eveyone, who is interested in the Automatisation of the installation.
It IS possible, even without a preconfigured and payed Client.
You need to execute these CMD Commands to generate the Config File needed for the automatised installation:
@echo off
echo [General] >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo clientVersion=3.2.1stable-Win64 (build 20210428) >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo updateSegment=86 >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo >> C:\Users%username%.ROLFBENZ\AppData\Roaming\Nextcloud\nextcloud.cfg
echo [Accounts] >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\localPath=C:/Users/%username%/nextcloud/ >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\journalPath=.sync_c58c51b03497.db >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\version=1 >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\targetPath=/ >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\url=SERVERURL >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\paused=false >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\dav_user=%username% >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\ignoreHiddenFiles=false >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\serverVersion=20.0.9.1 >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\webflow_user=%username% >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\virtualFilesMode=wincfapi >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\authType=webflow >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\version=3 >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\FoldersWithPlaceholders\1\navigationPaneClsid=@Variant(\0\0\0\x1e\xab\xea\xcf\x46*\xd7MU\x8f\xef\xb1\x31\x5\xf1|\xf3) >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo 0\user=@Invalid() >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo [Settings] >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo geometry=@ByteArray(\x1\xd9\xd0\xcb\0\x3\0\0\0\0\x1\x88\0\0\0\x37\0\0\x6z\0\0\x4\x4\0\0\x1\x89\0\0\0]\0\0\x6y\0\0\x4\x3\0\0\0\0\0\0\0\0\a\x80\0\0\x1\x89\0\0\0]\0\0\x6y\0\0\x4\x3) >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
echo >> C:\Users%username%\AppData\Roaming\Nextcloud\nextcloud.cfg
!!Replace SERVERURL with your personal Nextcloud-Url, Only works for the Client Version 3.0 and above(because Virtual Files are not available in V2.X) and has to be executed with the user, using the Client(not an admin) And the Username on windows has to be the same as the Username on Nextcloud!!