About suffering, expectations and unfalsifiable beliefs I've been wondering if it's possible to "progress" towards enlightenment if one has certain set of beliefs which may (or may not) be false (or illogical), but which do not generate conflict between expectations and experience. If we understand dukkha as the result of a dissonance between worldview and experience, one could a priori think that unfalsifiable theories may not generate such dissonances (and therefore, may not contribute to the perpetuation of the conditions of dukkha), because there's no experience that can disprove such notions.

For example, if one says that "our true self is formless, without comprehensible features", how could this view be a hindrance in the path? After all, that belief does not seem to necessarily contradicts the essence of anattā, because no khandha can be considered that alleged "true self", and so, non-attachment to khandhas may also be the path to attain that "true self".

Being more general: can someone attain Nibbana while still holding some false notions about self or reality?

Kind regards!
The Buddhist problematic isn't about whether beliefs are true or false, or provable or unprovable. It's about whether we cling to beliefs, because the clinging itself creates dukkha. 

So for instance, let's take your statement: "Our true self is formless, without comprehensible features." You adopt this belief, and like it, and it doesn't seem to interfere with your meditation or other practices. Fine. But then along comes another Buddhist, who says: "No, no no: that's not right.  What I believe is this...". Suddenly you find yourself involved in an intellectual dispute, wrestling (good-naturedly) with another practitioner about whose perspective is better, and which of you has more experience to back up their view. One way or another (unless you're very careful) someone's ego is going to get bruised. And then along comes some decidedly less enlightened person who says: "You two are a pair of blowhards and it's all a crock of sh*t anyway, so stop pretending you're so high and mighty." So not only did your statement make you a target for abuse, it gave this poor guy something concrete to vent about.

Fixed beliefs — even innocuous-seeming ones, and particularly ones that are not otherwise decidable — are focal points for conflict. Just think about the Christian faith, where a fixed, undecidable belief in a loving God indirectly fills the world with anger, resentment, and hate. I'm not going to go so far as to say that a belief like this is an obstacle, because beliefs like this are useful tools at certain stages of spiritual development. But in the end, even beliefs like this will have be released.