I am having an issue with new users on 18.0.3. When a new user logs in for the first time it says internal server error. Log in again and everything is fine. My data directory in on an NFS share and I use a symlink to in the nextcloud root to direct people.
The output of your Nextcloud log in Admin > Logging:
Error index OCP\Files\ForbiddenException: Following symlinks is not allowed 2020-04-08T13:39:56-0400
Error core Following symlinks is not allowed ('/var/www/cloud.hessercan.com/nextcloud/data/mgates/.' -> '/mnt/cloud/data/mgates/' not inside '/var/www/cloud.hessercan.com/nextcloud/data/mgates/') 2020-04-08T13:39:56-0400
Error core Following symlinks is not allowed ('/var/www/cloud.hessercan.com/nextcloud/data/mgates/files' -> '/mnt/cloud/data/mgates/files/' not inside '/var/www/cloud.hessercan.com/nextcloud/data/mgates/') 2020-04-08T13:39:56-0400
Error core Following symlinks is not allowed ('/var/www/cloud.hessercan.com/nextcloud/data/mgates/cache' -> '/mnt/cloud/data/mgates/cache/' not inside '/var/www/cloud.hessercan.com/nextcloud/data/mgates/') 2020-04-08T13:39:56-0400
Warning core OCP\Files\ForbiddenException: Following symlinks is not allowed