What is the relationship between saying that doing something is impossible and that understanding something is impossible?

Is saying that understanding something is impossible simply thinking that mysteriousness is a property of objective rather than subjective reality? In this case, it would be infinitely worse than simply saying that doing something is impossible, as it is a type error.

At the same time, the two seem related as they both may involve mistaking the limits of imagination for the limit of possibility.

Perhaps claims that understanding is impossible are sometimes from the first, more fundamental mistake, and sometimes from the latter. Alternatively, perhaps in practice all such claims draw from both errors.