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.

1. Make sure you're using the latest versions of Zotero and your word processor.
2. 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.
3. 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.
4. 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.
5. Make a copy of your document — by duplicating the file itself, not by copying and pasting the content — to use for debugging.
6. If using OneDrive on Windows, save the copy of the document to your local hard drive. OneDrive is known to interfere with the plugin for some people. If this fixes the problem for you, you can help us debug this conflict by posting to the Zotero Forums.
7. Open the copied file and check if you get the error after switching to a different bibliography style.
8. If the document has a bibliography, delete it and check if you still get the error.
9. 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.
10. Isolate the problematic citations. 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. Repeat the halving process on the section that fails, or pick one at random if both do. Continue this until you find the smallest possible section, ideally with a single citation, that must be present for the problem to occur. Remove the isolated citations from the original document and the problem should go away (unless there are multiple broken citations, in which case you'll need to repeat the process). Unless the error still occurs if you completely clear the contents of the document, this final step will by definition identify the problem.

If you encounter a broken document, please create a new thread in the Zotero Forums so we can attempt to fix the issue. Be sure to include a Report ID from Zotero, your operating system and word processor versions, the steps you have attempted to fix the error, and whether you were successful. You should also send the document excerpt from Step 10 and a link to your forum thread to support@zotero.org so that we can take a look.