Result was that all ID3v2 information in the MP3 list disappeared.
You have two main views. [Menu]-[Show Tags]-[Show ID3v1] or [Menu]-[Show Tags]-[Show ID3v2 tags]. You can switch between them fast with hotkeys Ctrl+1 and Ctrl+2. When you cleared ID3v1 tags list gets empty but if you switch to [Show ID3v2 tags] with Ctrl+2 you can see your ID3v2 tags. So tags are not lost.