I am having the exact same problem for the first time. I can access my new files on the computer where I add them to Zotero, but when my other computer syncs I get this new error message saying "The attached file could not be found. It may have been moved or deleted outside of Zotero, or, if the file was added on another computer, it may not yet have been synced to zotero.org." I have been using the same workflow for months without problems, so something must have changed on Zotero's end. Any assistance would be appreciated. Thanks!

Resolved. Saw error beside first computer sync, so hit sync. It "worked." Then synced a few more times, waited, and an error appeared again. Then synced again. Now it works. Not the greatest user experience, but all good now.