[JavaScript Error: "Component returned failure code: 0x80520012 (NS_ERROR_FILE_NOT_FOUND) [nsILocalFile.copyTo]" {file: "chrome://zotero/content/xpcom/translation/translate_item.js" line: 1127}]

Can you provide a Debug ID (different from a Report ID) from Zotero for an export that causes this?

You should be able to work around this (and help us debug this, if the debug output doesn't contain enough info) by narrowing this down to a single item that causes the error. You should be able to do that easily by going in halves — export half of the items at a time, and if one half fails, export half of that failing half, and so on. You should be able to find the item that's causing this in a small number of steps.

There's likely something weird about an attachment on the item that causes this — either that it's missing (though that shouldn't cause this) or that it has a very long filename or something else.