Asking as solution to work on an offline folder is not actually a troubleshooting solution
If using OneDrive on Windows, save the copy of the document to your local hard drive, or try renaming the file to remove any spaces in the filename. 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.
May it be a bug? Until few days ago Zotero worked well both offline and online.