**Same error (alert). PHP Version 7.1.26-1+ubuntu18.04.1+deb.sury.org+1 (Server API - Apache 2.0 Handler)
NC installed in /var/www/nextcloud
got it after upgrade to 15.0.5 from 15.0.4
Worked solution for me - Your web server is not properly set up to resolve "/ocm-provider/"