I think we're talking about different things. I'm not making any claims about the nature of knowledge, but rather justification. Surely you don't deny there is a problem there?

Justification is part of the nature of knowledge.

All that justification is, by the way, is "what S (the person in question) considers good, sufficient reasons for asserting that P."

The Münchhausen trilemma is about proof. Proof isn't something to worry about for reasons I've explained a few times.

paralogism

paralogism

With respect to your question, especially as it relates to Descartes, please take a look at what I believe to be two relevant topics I posted on TPF entitled "The Nature of Descartes' Proposition" and "Scope and Limits of the Invalidation Effects of a Defective Cognitive Nature on the Cogito Sum."

So, do you think that the thought "I exist" can possibly be false? If you agree that it cannot be false, then you agree that there are some thoughts we can't be mistaken about, surely?