I use Nextcloud since a few years and I am very happy with it (especially with the desktop integrations, e.g. KDE/Dolphin). I even use the Nextcloud WebDav serv

I use Nextcloud since a few years and I am very happy with it (especially with the desktop integrations, e.g. KDE/Dolphin). I even use the Nextcloud WebDav server as a backend to store and sync data for my own Progressive Web App :D The only security flaw I see is that it is written in PHP, which makes it harder for the devs to write secure code, but not impossible.