403 error client does not have permission to get URL

Hi Anya, 

Welcome to Box Community and glad to assist! 

If the user can't access your URL link, can you have him/her to log in through  https://app.box.com/login? 

If it works, please have your invitee to accept the collaboration invite. 

To find and resolve a pending invitation:

In the left menu of your Box account window, click All Files.
In the pending invitations panel under ACTION REQUIRED, hover on the name of a file or folder.
To see details about the invitation, click the ellipses, "...", and select View Details.
To decline the invitation, click the ellipses, "...", and select Decline All.
To accept the invitation and gain access to the project's content, click Accept.

Hope it helps!