Since Desktop App 7.0, the client only connects to ownCloud Infinite Scale — support for ownCloud Server (ownCloud Classic / oc10) was removed. If your client was upgraded to 7.1 (by a Linux distro update, or a manual update on Windows/macOS), your ownCloud Server account will no longer connect, and simply reinstalling 6.x won’t work: the client refuses to start on a configuration written by a newer major version.
The good news: the client automatically backs up your configuration before each upgrade, so your original 6.x setup can be restored without re-adding the account or re-syncing.
I’ve written up the full background and step-by-step recovery (for Linux, Windows and macOS) here:
And a reminder for everyone: read the release notes before upgrading — Releases · owncloud/client · GitHub — they flag breaking changes like this one.