Hi Ken!
I’m sorry you are experiencing that issue. Let me help you troubleshoot it.
First, please make sure you are using the latest versions of our plugins. Remember you can always download our plugins from your downloads page > https://theeventscalendar.com/my-account/downloads/
If you still experience the error with latest versions, Could you please let us know if after getting the error message, the attendees moved to the other event anyway?
Also, please share with us your system information by following this guide > https://theeventscalendar.com/knowledgebase/sharing-sys-info/. We’ll see if there is anything strange that would indicate an error from there.
Let us know about that.
Thanks,
Victor