Changed from Owncloud to Nextcloud. Completly new install, installed packages from Software Center (Snap), Server is working, as I can access from browser. When trying to point local sync folder to /media/Partition/Files, which is my partition for files, connection is created, but Nextcloud-client says: "Can't start sync journal". Sync fails. Owncloud was able to sync that folder. Is it a problem with snap?
No solution but a workaround. Found out that Owncloud client does the job. Nextcloud is a fork of Owncloud and for the actual version from Ubuntu Software Center (12.0.4 on Ubuntu 17.10) owncloud client has no problem syncing with Nextcloud server. Strange, but for now it is ok, I think in further versions it will might be different, as Nextcloud development seems to be faster. So, finally Owncloud client shows up all available folders and partitions, Nextcloud client doesn't, for example /media is not there for some unknown reason.