Good afternoon David and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about this message creeping up on your site.
I would love to help you with this topic.
I believe this is very similar to the following thread: https://theeventscalendar.com/support/forums/topic/conflict-with-event-calendar/
In other words there seems to be some type of conflict with the Testimonials plugin.
As you can see, it can be worked around, but I would recommend contacting the plugin author to let them know.
Let me know if that helps.
Have a great day!
Geoff B.