Thank you for your response. I have restarted multiple times, both the browser and Zotero, but it has not helped. I get the message "Zotero is updating your document. Please wait..." but after many minutes, it does not complete the update. Is there anything else I can try?
Upgrade Storage
If so, make a copy of the original document (File → Make a Copy) and see if it works there, and if not see the last step in Troubleshooting Errors in Word Processor Documents.