Steam on macOS is not a great application. I’ve been having trouble with cloud syncing failing for months now. This is particularly annoying as Steam will attempt to perform a pending cloud sync prior to launching any game, and you just have to wait for the sync to timeout before the game launches.

It turn’s out the problem is an interaction between Steam and iCloud Private Relay. Turn off Private Relay, and cloud sync works again. This was discovered by redditor “kaosimian” over two years ago, and the issue remains unresolved. But I guess that’s just what we should expect from Valve.

I’d rather have private relay so I’ve resorted to turning off Cloud Sync instead. Turning off Private Relay temporarily and then retrying all the cloud sync failures cleared some queue inside Steam, and then I was able to reenabled Private Relate.