Error message when following Calendar Notification Link.
 

I have a member who is receiving our calendar notifications, and when she clicks on the link she gets ane error message that says the link is not valid. She replies to the email notification telling me what is happening, and I am able to click on the link and it works.

Any ideas what might be causing this, or how I can fix it? Could it be her Firewall? No one else is having this issue.