I didn't say the original is incorrect. I meant that if an uncountable abstract noun is modified by an adjective (not necessarily by the way), it could turn into a countable noun. Don't you agree with that?
This version with "an" would be very appropriate if the writer then went on to describe what distinguishes this particular form of open-mouthed incredulity from any other kind of open-mouthed incredulity:
The vicar stared at him with an open-mouthed incredulity that amused the congregation no end.