Good evening Thomas and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the error message you are receiving while attempting to create a ticket.
I would love to help you with this topic.
I believe you are absolutely right. This type of message is usually being produced when there is a version mismatch in the code. By looking at your plugin list, I am noticing that Events Tickets Plus was one version behind (4.0 as opposed to 4.1).
Can you confirm if updating that plugin gets rid of the message ?
Best regards,
Geoff B.