Hi Martin,
I just installed nextcloud 11.0.0 on one of my Plesk Onyx servers. I did not change much on Plesk configuration except to switch to php 7.x for the webspace. Here are the steps I followed
1. login to the console as System user for the webspace
2. download the tar file from nextcloud.com, best to do is with wget
3. extract it to a folder in your target webspace, it extracts to a folder nextcloud
4. create a database in Plesk for the webspace
5. open the URL http://<yourspace>/nextcloud
6. finish the steps in the Web Wizard
7. Change the open_basedir and add "{:}/dev/urandom" at the end of the value
That's it, works fine
If you do not have shell access than you can extract the tar file locally and copy it via file manager.
Cheers
Jürgen