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This looks to me like a communications issue between her browser and the request to your server. Your hosting support might be able to shed some light on it but my guess is that this is more to do with your users PC setup and browser setup.

I would suggest to her that she first try with an alternate browser and also follow the recommendation in that message to clear her browser cache completely before she attempts to use it.

If, however, it is connected with not getting subscription emails then her problem is likely to be more to do with the setup of her machine operating system. Maybe she has some sort of problematic firewall or security sub-system in place that is  mid-behaving or not set up correctly.