Hi piotrcierkosz,
It seems that the error message is a Azure AD login error. This article give a explanation of the issue: http://www.agilepointnxblog.com/windows-azure-active-directory-login-error-aadsts50020/.
It recommends you delete your Teams client cache as the following steps:
- Fully exit the Microsoft Teams desktop client. To do this, either right click Teams from the Icon Tray and select ‘Quit’. Or run Task Manager and fully kill the process.
- Go to File Explorer, and type in %appdata%\Microsoft\teams.
- Once in the directory, delete any file in the following folders:
%appdata%\Microsoft\teams\application cache\cache
%appdata%\Microsoft\teams\blob_storage
%appdata%\Microsoft\teams\Cache
%appdata%\Microsoft\teams\databases
%appdata%\Microsoft\teams\GPUcache
%appdata%\Microsoft\teams\IndexedDB
%appdata%\Microsoft\teams\Local Storage
%appdata%\Microsoft\teams\tmp
If it is no luck after that, you could uninstall and reinstall the Microsoft Teams client and have a try.