Translation:She loved mysteries so much she became one.
I'm not a native Dutch speaker so maybe somebody who is can correct me here if I'm wrong, but it seems that "er" is always necessary when referring to a number of something where the something is only implied.
So for example:
"I saw two trees" = "ik zag twee bomen"
"I saw two (of them)" = "ik zag er twee"
"She still has one shoe" = "ze heeft nog één schoen"
"She still has one (of them)" = "ze heeft er nog één".
Hope that helps :)