When I upgraded from owncloud 9 to Nextcloud 11, my web interface was broken and I could not login to Nextcloud with the web interface. I was able to sync files to the server. I was recieving “Internal Server” errors and the web interface was broken and unusable. I documented the steps to fix the web interface over at my other blog
https://blog.chadchenault.com/2017/12/30/nextcloud-12-internal-server-after-login/