Good evening Andreas and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the message you have been receiving.
I would love to help you with this topic.
I am not sure exactly why you are receiving this message, but it looks like somehow the system believes that the license key is still tied up to the old web host.
I would recommend reading the following article on that topic: https://theeventscalendar.com/knowledgebase/moving-your-license-keys/
Let me know if that helps.
Have a great day!
Geoff B.