Note the difference:
À cause de diverses formalités liées au décès de son père a Bruxelles, elle était obligée de séjourner encore quelque jours dans la capitale belge - ce qui arrangeait deux policiers.
Because of various formalities related to the death of her father in Brussels, she was obliged to stay for a few more days in the Belgian capital - which was convenient for two policemen.
À cause de diverses formalités liées au décès de son père a Bruxelles, elle était obligée de séjourner encore quelque jours dans la capitale belge - ce que deux policiers arrangeaient.
Due to various formalities linked to the death of her father in Brussels, she was forced to stay for a few days in the Belgian capital - which two police officers arranged.
In the former:
Qu'est-ce qui arrangeait les deux policiers ?
In the latter:
Qu'est-ce que les deux policiers arrangeaient ?
Update: There is a nice comment below that I wish to add to my answer. It provides better insight to your query. (Many thanks to the teacher of French @Luke Sawczak for his comment and the verification as well!).
Your mistake is actually one of vocabulary, not grammar. There is a verb arranger that means to suit someone or to please someone. Thus the officers didn't arrange her stay, in which case they'd be the subject of arrangeait and you'd be right — rather, her stay suited the officers. You can find a clue as to the true subject in the conjugation of arrangeait, which is singular. Your original reading would actually require two errors, not one.