Hi Jan,
Thanks for reaching out!
It looks like you might have some customizations running. If so, can you please share them with me in a private reply? Is this occurring on every ticket, or just certain ones? Finally, is there anything else that looks broken in the email, or is it just this warning?
I think it would also be useful to try testing for conflicts with your other plugins, which you can do by following these instructions.
There could be a potential issue here that we need to address on our end, so providing that additional info will help me determine the source of that message and work on finding a solution for you.
Thanks!
Jennifer