I haven't experience this before myself in Quick Base, but 403 is an HTTP status code that a server may return to a client to communicate that access is not authorized for some reason. It's odd that this shows up and it didn't just redirect the client to a login screen or some other user-facing error screen.
You might consider checking the link that Quick Base sent to your users to see if it's malformed. If the issue persists, I'd create a support ticket and reference the transaction ID. Someone at Quick Base should be able to use that information to provide you more context on the error.
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Nathan Hawe
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