Nothing has changed in Zotero, but there's currently some issue with file sync requests to Amazon cloud servers for some people in China — we've gotten many reports of this in the last few days. If you can you provide a Debug ID (different from a Report ID) for a sync that fails, we may be able to say more, but this is likely some problem with your internet connection.
Unfortunately, it looks like, for some people in China, file transfers to and from Amazon cloud servers are either being interrupted or timing out after 30 seconds of inactivity. Other requests to our servers seem to be working fine.
All we can suggest at the moment is trying from a different network or waiting and hoping these requests stop being blocked/broken.
Our workaround last week fixed file-sync downloads that were getting blocked in China. We've now added a workaround for blocked uploads as well. If you're still having trouble, please provide a Debug ID.
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All we can suggest at the moment is trying from a different network or waiting and hoping these requests stop being blocked/broken.