subscribe option suddenly stopped workinb

Hi
I was alerted by one of our subscribers yesterday that his All in One Calendar option suddenly stopped working.

After checking both my phone and my tablet I found the same.
When tried to subscribe again I and the person alerting me to this have both, independently, received this same message

This email address isn't associated with an active Google Calendar account: https://m………../?plugin=all-in-one-event-calendar&controller=ai1ec_exporter_controller&action=export_events&no_html=true. Please check the email address and try again.

Tried numerous times without resolution

https://wordpress.org/plugins/all-in-one-event-calendar/