Just a quick update to the old threat, as Google points here:
The file is nowadays called core.entity_registry and it is a json file. Deleting in the configuration interface might sometimes be tricky. In my case, I couldn’t find the entity, because it was automatically created by another integration but also automatically disabled and thus not visible. I found it in the json file, where it also listed the corresponding integration. Going there in the interface I was able to find the entity and manage the entity without manipulating the json file by hand.