Error when trying to share a sheet with a specific email address



I have a long list of sheets that I've been going through and sharing with their owners. For some reason, when I type in one person's email address and try to share, I get "Error has occurred. Please try again later." This hasn't happened with any other email addresses.

It won't let me save this person in my Smartsheet contacts either. Has anyone had this problem / know how to resolve?