Temporarily switching to the alternative repository mirror fixed it for me (mentioned here Qt-5.6.2 everywhere · Issue #5470 · owncloud/client · GitHub). https://download.owncloud.com/repositories/desktop/Ubuntu_17.04
The following packages will be upgraded:
libowncloudsync0 ocqt562-libqt5core5a ocqt562-libqt5dbus5 ocqt562-libqt5gui5 ocqt562-libqt5keychain1 ocqt562-libqt5network5 ocqt562-libqt5opengl5 ocqt562-libqt5printsupport5 ocqt562-libqt5sql5 ocqt562-libqt5sql5-sqlite ocqt562-libqt5widgets5 ocqt562-libqt5xml5 owncloud-client owncloud-client-l10n
14 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.