If you can insert Zotero citations into new documents but receive an error in an existing document, follow these steps to find the cause of the error. Whether you manage to resolve your issue or not, please submit a Report ID from Zotero and create a new thread in the Zotero Forums with the Report ID, your operating system and word processor versions, the steps you have attempted to fix the error, and whether you were successful.

1. While troubleshooting, disable the Track Changes feature in Word/LibreOffice, as it can have complicated effects when working with Zotero. If Track Changes is enabled when you insert or modify a Zotero citation, it may mark many or all of the Zotero citations in your document as changed or cause field codes to be displayed. On rare occasions, Track Changes may cause Zotero to think a citation is corrupted. If you had Track Changes enabled previously, try accepting all changes to see if that resolves the issue.
2. Check for citations in image captions. Zotero won't let you insert them, but if you copied a citation to a caption that's most likely the source of the problem. Delete it.
3. Try copying and pasting the document content into a new document to see if the problem goes away. You may need to click the “Set Document Preferences” button before your old citations will be recognized.
4. Make a copy of your document — by duplicating the file itself, not by copying and pasting the content — to use for debugging.
5. Open the copied file and check if you get the error after switching to a different bibliography style.
6. If the document has a bibliography, delete it and check if you still get the error.
7. While debugging, if you are using Fields mode in Word or Reference Marks mode in LibreOffice, it may help to display field codes rather than formatted text. To do this press Alt/Option-F9 (or Alt/Option-Fn-F9) in Word or Ctrl-F9 in LibreOffice.
8. Try to isolate the problematic citation. Delete half of the document at a time and see if the error still occurs. If not, use Undo to restore the deleted section and then try deleting the other half. If you can reproduce the error in only one of the halves, repeat the halving process on the remaining section. Continue this until you find the problematic citation. Remove that citation from the original document and the problem should go away (unless there are multiple problematic citations, in which case you'll have to repeat the process). If you are able to find a problematic citation, you should also send that document excerpt to support@zotero.org so we can take a look at it. For all other discussion, please post to the Zotero forums.