I created a new view in Cloudbees and, while configuring it, wanted it to remain somewhat unnoticed by the rest of my team. I renamed the view to "..", just because it was the first thing that came to mind.
Now I can't delete the group, because the use of ".." is interpreted as directory traversal.
Just as I was finishing this question, I had another idea which was successful. I found by accident that "https://{account}.ci.cloudbees.com/view/../delete" will delete the default view. Another test confirmed that "https://{account}.ci.cloudbees.com/delete" deletes the default view.