This means Zotero is trying to download one of your attachment files from its cloud storage, but the connection is interrupted. This is a network-level block, not a missing file. Zotero retries and hits the same wall every time, which triggers the sync error message.
The most common cause is a firewall, security software, or network restriction blocking Zotero's connections to its cloud storage.
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The most common cause is a firewall, security software, or network restriction blocking Zotero's connections to its cloud storage.