not all cards on the Kanban board are visible

@Nina Stumpp - This is definitely unexpected behaviour and not a feature.

There is a known bug re not being able to move/delete Cards - this could be a side effect of whatever is causing the other issues.

I few things your users could try:

1. signing out and back into Miro
2. clearing the browser cache
3. disabling all browser extensions (a private/incognito window is usually a quick way to accomplish this)
4. Try the Miro desktop app