Hello Tamas,
In order to narrow down the issue and help you better, I want to know if you are using Teams desktop application and
Teams web application.
If you are using Teams desktop client, please sign in Teams web application to check whether you are having the same error message. Sometimes the issue may only happen in Teams desktop client. Under this circumstance, please clear the
local cache and sign in Teams desktop client and sign in again. Please go to
%appdata%\Microsoft\teams\Application cache\Cache, delete all the files. Then go to
%appdata%\Microsoft\teams\Cache and delete all the files.
If the same error message shows both in Teams desktop client and web application. Please let the admin remove you from the Team, wait for a hour and re-add you as Owner to see if there is any difference.
Please feel free to post back when you have any update on it.
Regards,
Rick