A friend of mine asked me once what he could say if he was asked about a colleague or a friend not being able to attend a meeting or a party because of, for example, confidential health or family problems, or any private matters. What he wants to say -without being specific or giving any reasons-is something like
He has some personal circumstances"
It's unlikely that the phrase "personal circumstances" sound natural. So what would native speakers "vaguely" say in a situation like this. I hope it is clear that the question is not about how to give any suitable answer but rather about giving an idiomatic one that is as close in meaning as possible to the matter at hand.
You could ask for more clarification in the comments if I was not clear enough.