Before opening a support ticket, I tested deletion today morning once again. Now, I could delete the organization. But: My account was deleted, too. I was logged out and I could not login again. I tried password recovery, but I did not receive an email. Now I created the account again. Same username/email address, same password, another organization. Apparently, the account existed, I had to login with my 2FA TOTP which I set up before. After switching to another of my organizations, I was logged out, again. I could not login. So I had to create my account, again. Same procedure. The fourth try was successful. I created the account with an existing organization. The first three times, I created new orgs. Now I am logged in, the CatC organization is deleted.
Very strange behavior.