I use iCloud Photos Library to store my original photos in the cloud, to save space on my Mac. Today I tried to download the full resolution version of a few of my older photos and they refused to download, coming up with an exclamation point error on screen and telling me to try again later.
Checking the console log didn’t reveal much: just a message from cloudphotod about the sync session/download failing. No explanation of why, just some error codes – a different error code for each failed download.
I dug around on the Internet and found a way to supposedly fix the photos sync, by using the Photo Library Repair Tool. This tool is built into the Photos app: Hold down the Option and Command keys while launching Photos. Then follow the prompts:
This worked great for me. I was able to download my old photos again in full resolution after the repair was complete. It took a few hours, but I was able to use my computer in the meantime. Note that your search index for photo keywords will need to be rebuilt, so you will need to wait for that too if you want to search your photos. This search index rebuild happens automatically, in the background.