I have been running into this issue on a full server build. I am able to bypass the error by editing util.php located in the nextcloud directory under lib/private/legacy/. Lines 1015 to 1020 can be commented out and the file saved. Then try logging into the server through your browser. This is the only temporary fix that I have found for this issue. With the latest update of Nextcloud 15.0.5, all of the Nextcloud files are hashed. So you will get an error that the util.php file has been modified.
Some other things I have done is create a symbolic link owned by the web-server user in the nextcloud directory for data that points to my share. I researched to see if it was possible to change the permissions of symbolic links so that only the owner and group could access it. Alas, also no.
I tried creating another directory inside the share that was owned by the server to have all of the data files. That also did not work.