If possible choose a subcategory matching your languageI have Owncloud 9.1 installed on a centOS 7.3 server which works correctly from the web browser and from the smartphone app. The problem comes when I try to connect to the server from the desktop application (mac windows) this will not connect showing a connection error rejected by the server ....
If the browser and the smartphone app is working then you're most likely using an outdated version of the sync client which is blocked by default from accessing recent ownCloud versions.
Nothing is obvious if you don't provide this information. See the following thread only posted a few days ago where some one had used 1.5.4 released 3 years ago...