The nextcloud macOS client is an annoyance. All my macOS computers suddenly stopped syncing, and the client gives me no explanation why it cannot connect anymore. That’s not the first time that these kind of problems occur - it seem the client needs more testing and a better error handling before it is released.
It cannot be the solution to always install the owncloud client as @jakobssystems said just to keep the system running - nevertheless I gonna do it again - hopefully the last time, because it is really annoying.