I imagine the context is something like "the associated permissions were set by a different group or department within Google and/or decided by software running elsewhere that didn't communicate its reasoning as part of the protocol it speaks to the load balancer". That is, that's all the system generating the error message knows.

But yeah, it also feels like "we (Google) don't care that much about your problem here, because nobody is going to improve this software to give you a more meaningful explanation".

often disclosing any more information is considered a security leak

I imagined it being sort of tongue in cheek, like saying "who wants to know?". _That's all we know_ minimizes alarm to laymen while communicating that the page contains no other interesting information.