Yes thank you all solved and understood.
- The original local administrator is created at cluster bootstrap.
- There is no “default local admin” username.
- The original local admin is not showed on the Web UI.
- It is possible to create a local user with admin(/other) rights from Web UI with the same name as the original local admin:
- deleting this user from Web UI does not delete the original local admin (of course, but I wondered, I tested, no problem)