The two most common error messages people encounter with Zotero are:
This error message can occur with browser connector, or when adding source information in Microsoft Word, Google Docs, or LibreOffice/OpenOffice.
If the desktop app is open and you are trying to add an in-text citation or bibliography
If you've done some editing and deleted in-text citations, there may be lingering field codes that are hidden.
Occasionally, when using the Browser Connector to add sources to your Library a 'not responding' message appears. This typically happens when you are downloading multiple sources with pdfs at once or the pdf is very large. Just leave the Connector and desktop app alone to resolve on its own. If it seems stuck after several minutes, it could be an internet connectivity issues or an issue with your computer's processing power. Close your browser and try again. Or close all apps on your computer and restart your computer.
If you are not able to find an answer to your issue here, the Zotero community has a robust knowledge base and discussion forums. Most answers to issues can be found in there.