I can confirm that this problem exists in both High Sierra and in the beta release of Mojave. While I have managed to get it to work in Messages for macOS (both said versions) on my MacBook, it does not work on my iMac in either installation (High Sierra or Mojave beta): instead I have the exact problem of the original poster: in Messages on macOS it simply indicates ‘Downloading Messages from iCloud...’ and a status bar beneath the messages list in the window, and then does nothing. It will hang this way for days at a time; and my network activity monitor confirms that during that period, no data is actually be transferred — so it says it is downloading, but nothing is actually downloading from the cloud (i.e. it is not a server load or bandwidth issue).
For reference, I have already:
- Rebooted the system, soft and hard
- Disabled and re-enabled cloud sync for messages in Messages > Settings
- Disabled and re-enabled the entire iMessage/iCloud account in Messages > Settings
- Disabled and re-enabled iCloud in System Preferences > iCloud (and to confirm, all other iCloud services are working fine, including desktop and documents sync, photo sync, etc.).
- Disabled and re-enabled iCloud message syncing on my iOS device.
- Completely re-formatted the hard drive and done a completely new installation of “Macintosh HD” as well as the installation of macOS and upgrade to Mojave beta.
None of the above has provided any help in getting the sync to work in messages. I continue to see the “Downloading Messages from iCloud…” slider at the bottom of my message screen, with the blue status bar, while nothing downloads or loads. As before, this occurs both in the High Sierra installation, as well as the Mojave beta,
As this is touted as one of the major new features of macOS, I think a swift fix is necessary.
I was just on the phone with a tech support supervisor for an hour, and received no workable fixes there. I have also filed a Feedback ticket in the Mojave beta for this problem.