After upgrading to nextcloud 16.0.5 it seems that ios clients might have a problem logging in.
It worked fine until after the upgrade where the client loops on login failure.
{"reqId":"7ltiIUPQe5YbTVnSilrt","level":2,"time":"2019-10-02T08:47:44+00:00","remoteAddr":"x.x.x.x","user":"--","app":"core","method":"PROPFIND","url":"\/remote.php\/webdav","message":"Login failed: 'xxxx' (Remote IP: 'x.x.x.x')","userAgent":"Mozilla\/5.0 (iOS) Nextcloud-iOS\/2.24.1","version":"16.0.5.1"}
Here is a github issue: https://github.com/nextcloud/ios/issues/947