This post by Ebeling on Coordination Problem is very interesting and well worth reading.
Mises's epistemology was characterized by a dichotomy: Theory and History. The epistemic properties of the two realms of social inquiry were opposite. I think that it is better to use a Husserlian terminology and call them "research of essence" and "research of existence": the former, Theory, is about logical relations among concepts, the latter, History, is about all the complications which arise when it comes to apply Theory to the real world.
The hyperrationalist interpretation of Mises, which I think was due both to Rothbard (in which constructivism and apriorism play important ideological functions) and Hayek (for instance, when he said that Mises's "Socialism" was a great and influential work, but too rationalistic), is not compatible with what Mises writes, with or without the additional important insights which Prof. Ebeling has added with his post (which I'm looking forward to reading in the referenced books by the Liberty Fund).
Without History, what remains of Mises is a delusional hyperrationalist who thinks to be able to derive propositions about the real world out of purely logical constructs. This interpretation is not consistent with Mises's writings, in which the dichotomy between Theory and History hides all the uncertainty of the real world in the latter camp. Mises claimed explicitly that the apriori may have had an evolutional origin, that all theories are incomplete and subject to radical changes, that all the rational constructs are based on more or less arbitrarily chosen "ultimate givens". Where's the hyperrational? To have clear ideas and to state them clearly cannot be a symptom of hyperrationalism.
Let's make an example: socialism is impossible because there cannot be monetary calculations AND monetary calculations are necessary for coordinating a complex economy. The last hypothesis is not apriori: it is an empirical generalization of very vast applicability.
There are cases in which empirical hypotheses, which are logically necessary to apply Theory to History (essence does not beget existence, Saint Anselm notwithstanding), are not as clearcut. While it can be claimed that reality is complex, production takes time, capital is heterogeneous and information is scarce without fear of being falsified on a posteriori grounds, it cannot be said with the same level of certainty that the structure of production is sustainable or not, if a boom is fundamentally sound or will be turned into a bust, whether economic agents are conned into believing, or acting as if, something is true when it is not...
Relevance is never a priori concept: it is an a posteriori judgement about the relative importance of several conceivable causal factors in the interpretation of a specific historical event.