Error starting Mac Standalone
This discussion was created from comments split from: Zotero won't start: "There was an error starting Zotero.".
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If that doesn't fix it, you could try installing the Zotero 5.0 Beta to see if that helps.
Beyond that, see the steps in Reporting Startup Errors for additional info we'd need.
The recommended step of deleting the Zotero Cache seems to have fixed the problem for me.