"I am not an expert in this matter, but my understanding was that XYZ should have happened. Could you tell me what the reason was that you did ABC instead, so I understand better what is going on? "
So you state the assumption that the person did the right thing, but you don't know why it was the right thing, and you want to know why so that you understand the situation better. That's absolutely fine to state, and anyone would give you a nice explanation unless they have severe personality problems.
If the person did indeed make a mistake, that gives them a chance to pretend that they found the mistake themselves (and not you), and admit to it or even start fixing it before you realise that it was a mistake - which is ten times better than being told by you.
(This will be different from country to country, but some state agencies may be a lot more helpful if they get a call from your lawyer / tax advisor etc. who says "I messed this up for my client, how can we fix this", because they don't like punishing you for someone else's mistakes).
PS. The question was "How to ask them without making it look like a personal attack", and that's what I replied to. Now if someone messed up, as that person apparently did, then "putting up with passive-aggressive nonsense" as it has been called is the smallest of their problems. The way I wrote this question they have a chance to explain what happened, maybe even fix what happened, without losing face. If that is not what the person wants, that's up to them.
Some people don't like arguing. That doesn't mean they keep employing you. It means they may fire you without arguing. Instead of "You're fired, you lazy idiot" they may say politely "I think it is better if our ways should part". The effect is the same.