That's not a relevant error. This is generally just an intermittent issue with Amazon S3, which is used by Zotero File Storage. Just sync again, as the message you're actually getting ("File upload failed. Please try again.") says.

I'm getting the same error and have been ever since I bought additional Zotero storage space and switched to syncing my libraries there. (The report number is 1505888838.) I understand the advice to "just sync again," as Dan advised pwnorris, but roughly how many attempts does one have to make before getting some additional advice on a fix or workaround? I've rebooted several times, and re-installed Zotero.

pwnorris: Roughly how many attachments do you have? You can provide a Debug ID for a sync attempt that generates the error and we can take a look, but, unless you're getting this error immediately after clicking the sync icon, I suspect it's actually syncing a sizable number of files each time before Amazon produces an error and it just needs to get through them all. (Zotero should probably automatically retry after Amazon failures, but it doesn't currently.) You can hover over the sync progress bar to see progress.

Your socket connection to the server was not read from or written to within the timeout period. Idle connections will be closed.

This is the error you're getting, and it's from the Amazon servers. It likely indicates a network glitch on either your end or theirs, so I'm not sure what we could advise other than to keep trying.