A wrong key can break a page that worked a few minutes ago and you get the generic error:
"Unable to display this Web Part. To troubleshoot the problem, open this Web page in a Microsoft SharePoint Foundation-compatible HTML editor such as Microsoft SharePoint Designer. If the problem persists, contact your Web server administrator."
Solutions:
- Routinely create backup copies of the page for comparison and restoration
- SharePoint Designer likes to randomly add characters to your markup which will break the page. Search for and delete these.
- Never delete the original new form from a page. Hide it, but never delete it. This will keep the attachment functionality working correctly.