On latest macOS Safari 13.1 (14609.1.20.111.8) on macOS Mojave 10.14.6 Nextclouds’ web interface doesn’t work properly. For instance the login dialog is not shown. Web Console shows that scripts do not appear in the script-src directive of the Content Security Policy but they are and safari worked before.
The Nextcloud version is the latest 18.0.3, older versions seem to work as far I 've tested.
No server logs seems to help due the fact that this has something in common with the latest Safari Update Apple released yesterday.