Ok, so indeed that was the issue, I don't know why though, I have never changed that setting in my starter page, since I began using radzen, but this issue has come back multiple times. It seems something elsewhere on Radzen changes the setting from "Everybody" to "Autheticated" randomly without any specific reason at all.
Small suggestion: Maybe if a page is set as the Login page give a warning to the user if he tries to set it to authenticated access? I never would have checked that setting, and would probably pass over it.