@Paweł Wrzeszcz - I am able to reproduce the warning message you are reporting by putting a card in edit mode in one browser and then trying to delete the entire user story map framework as another user:

However, this scenario does not match yours.

Can you tell us a bit more?

• Is there anyone else on the board when this happens? (Check the participants pane):

  

• Are you using a browser or a Miro desktop app?
  • If a browser, which one?
  • Have you tried clearing your browser cache?
• Are you using Windows or macOS?
• Have you tried leaving and going back to the board and trying again?

Something lese I have never come across:

I was also able to generate another warning while trying to delete a release while another board participant has a card in that release in edit mode: