There is a distinction between those two expressions.

“de nouveau” has the meaning of “one more time” (e.g. On l’a emprisonné de nouveau) whereas “à nouveau” has the meaning of “one more time and differently” (e.g. L’enseignante a dû formuler à nouveau sa question pour que les élèves comprennent).

Nouveau

 

See also this post on the same topic.