Invalid email address error for multiple participants after they complete Zoom registration

[nm-zoomhost]

Our all volunteer non-profit uses my Zoom Pro with a large meeting monthly add-on to conduct public Zoom workshops.  Our MailChimp subscribers respond to our invitations by clicking a link that takes them to the Zoom registration page for the particular monthly workshop.  

Registration has never been a problem previously.  This time, for a mid-August meeting, registration is working for many but I have six different complaints now that Zoom is refusing to recognize valid email addresses.  One of the complaints is from a board member who routinely participates via Zoom.  

I tried to enter one person's registration information twice, with two different email addresses they gave me.  The same problem repeated for me, with both email addresses.  This person registered successfully for our July workshop with the same information.  They are a subscriber..

Registration is required.  Authentication is not required.  

Please help.  Is this a Zoom bug, or a setting that I've gotten wrong, or what?  

[nm-zoomhost]

Nevermind.  I rechecked authentication.  It was on.