Dear respected gronenthalr,
Good day! Thank you for posting to Microsoft Community. We are happy to help you.
I’m sorry you’re having a problem but based on your description we can work together to resolve it.
For the situation you encountered, we deeply understand the inconvenience caused and apologize for it.
As per your description, to check whether this issue related in Microsoft Team desktop client app itself, if possible, could you please access Microsoft Team for web(online) and see OneNote Class Notebook.
Meanwhile, could you please confirm does other users in your Microsoft Team class are also experienced the same behaviour? also, could you please confirm does this issue occur on a specific Microsoft Team class, it will help for us to narrow down your problem.
Please understand that our initial reply may not always immediately resolve the issue. However, with your help and more detailed information, we can work together to find a solution.
I'm sorry and I apologize for that.
Appreciate your patience and understanding and thank you for your time and cooperation.
Sincerely,
Waqas Muhammad