Issue in NC 13.0.0 (it will be fixed in 13.0.1), related github-issue -> https://github.com/nextcloud/server/issues/8043
Symptoms: White screen on web-interface (except for the navigation elements), other display issues with web-interface (also when individual themings are used).
Workaround: Delete the CSS and js cache on your server, run these commands in your nextcloud folder via terminal:
rm -r data/appdata_*/css/* data/appdata_*/js/* occ files:scan-app-data reload the page
Remark: If the whole screen is white, there can be as well other server issues, for more information you have to check your logfiles (webserver, nextcloud).