Client gets error message when she clicks on Paypal link

When I send an invoice, I include the Paypal link in the email. I've tested the link and it works for me. However a client says she gets this error message when she clicks the link:

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Zoho Invoice

The page you are looking for is temporarily unavailable. This can be
because of an unhandled error or network connectivity issue. We will
duly fix this issue.
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The same link works fine for me. What could be happening? She works at a major US corporation and is probably accessing the internet through some sort of content filter.

Thanks.