This occurs both when I try to sync and when I try to replace the server library. I have been using the javascript api to create items, including movement of items between libraries, and may have corrupted something, though nothing is obvious on my side - the database had some integrity errors that were cleaned - I don't know if this process would have created output of any kind. Even after cleaning, however, the sync error persists.

Full error is:
Error: Cannot set key after object is already loaded in Zotero.Item.setField()" {file: "chrome://zotero/content/xpcom/sync.js" line: 638}]