Well, I was fighting with this minor issue for almost a week now and as soon as I open a discussion about it, I’ll find the answer accidentally myself …
This is what I did:
There might be multiple items related to a Host (e.g. localstorage,…)
My mistake before was, that I went to Inventory browser --> Host System (within the Tree View). There I could choose the actual ESXi Host, but the Delete Object Icon was greyed out.