ooh, afraid I can't agree with you there Werner - I'm with you, Bingley. It is a question of how easily we can recognise where the phrasal clauses break down: does it parse as

“[All {of the required fields} are not] filled in?”
or
“All of the {required fields} are not filled in?”
or
“[All of the required] fields are {not filled in}?”
or
“All {of the required fields} are [not filled in]?”

or indeed some other possible variation!

Clearly the intention was to state that “Of the fields requiring completion, not all have been filled in”, but IMHO the actual text used represents an awkward displacement and clefting of the not and all, which just serves to introduce a degree of ambiguity: English word order is so important to our pattern recognition and syntactical emphasis since we got rid of all the other clutter We tend to pack the new lexical parcels at the beginning of sentences to give them prominence, so the sentence as constructed has the effect of highlighting the “ALL” rather than the “NOT”, which works against the required sense to be conveyed.


By contrast, we can find no possible ambiguity in the alternative phrasing of:

{Not all of the} {required fields} are {filled in}

because we will have all encountered many versions of analogous phrasing, such as

{Not all of the} {apples} are {red}