Oh my god, Tony. I am so sorry. I have been totally confused.
I thought I have to delete a category in the selection-field to remove the relation between an event and a category.
Now, creating a new event I recognized that all available categories are shown up and I have to choose in an active way which one to relate to the article. So I do not need or have to "delete" or "remove" a category, save the event in expectation that the deleted category is not seen any more.
I am so sorry. I am just a blockhead.
Wish you a merry, merry christmas and thank you again for your constant kindness and patience and willingness to help me.
Holley