There had followed then a time of such happiness that Chips, remembering it long afterwards, believed hardly it could ever have happened before or since in the world.

Explanation

The error is in the word order and adverb placement. "Hardly" is incorrectly placed, it should be used to modify the verb "believe" instead of "remembering". The sentence should read: "There had followed then a time of such happiness that Chips, remembering it long afterwards, could hardly believe it could ever have happened before or since in the world."