"She was at his side when he had problems."
Translation:Hun var ved hans side når han havde problemer.
"Hun var ved hans side da han havde problemer" means that he had problems at one point in the past, and that she supported him through those problems.
"Hun var ved hans side når han havde problemer" is more general and means that every time he had problems, she was by his side.
Thank you for your reply! So you're saying, that the difference is in the repetitiveness of the problems but both versions are correct translation because the English sentence is ambiguous? I actually wouldn't think of the English sentence as repetitive without every time at all :-).