Nextcloud doesn't sync any files, in spite of declared status

I finally got a Nextcloud 15 server installed on a Raspberry Pi (stretch), together with clients on the same Raspberry Pi, and a Mac (Mojave). I then created 3 files in the respective Nextcloud folders on Machine A & B, and at the server web site:

TestFileA on Machine A
TestFileB on Machine B
TestFileS on Server

Shortly thereafter, the Activities window in each case (clients and server) says that the file has been synced (complete with check mark), but in fact, each file exists only on the site where it was created.

So I have a system that claims it is syncing but in fact is doing no such thing.

Is there some setting to fix this?