Yeah, that stinks. You don't happen to have a screenshot of how it looked exactly? There are two kinds of error messages, the ones we write and the ones Duo generates automatically. I searched the list of our messages and there aren't any for this sentence. There is no accepted solution containing at the night and the sentence hasn't been changed for 8 months. So there's nothing I can change, but there could be some bug that I should report to Duo staff.
In some other material I use for learning it is stated that "under + period" refers the to the complete period, as in "Under 1800-talet var Sverige ett fattigt land", meaning Sweden was poor during whole century.
Unless I do not want to stress that the poor guy was in agony the whole night, shouldn't this be "på natten"?
Native here, from Southern California. I slightly disagree. "He died at night" simply emphasizes the nighttime. It's a proper response to a question like:
"Why didn't anybody call 911?" "He died at night. No one knew until the next morning. He passed peacefully."
Of course, 'he died during the night' could be more broadly used, when not emphasizing the time as much.
Interestingly, as usual, the shading of the meaning seems to work very similarly between English and Swedish, during the night = under natten; at night = på natten.
I feel like encountering Swedish sentences using the word "to die" and its different forms more often than not. Am I right? I understand that "to die or dying" is one aspect of our life here, so to speak, so we must learn how to be albe to express ourselves in this regard in Swedish too.