Die Nachricht belastete ihn.

English Translation

The news was a load of his mind.

Even if “of” is a misprint for “on” this isn’t idiomatic.

I think there’s two things that went wrong.

#1. I think it should be “The news was a load off his mind.”
See here: A load off my mind - Idioms by The Free Dictionary
It means “relief”.
Looking at tatoeba, the sentence is correct there. But it seems it has been from the beginning. So no idea where “of” comes from.

#2. The German translator at tatoeba did not get the meaning of this idiom and translated it wrong, with the German sentence unfortunately having the opposite meaning.