By cassette holder, I assume you mean the cassette lockring. If so, the required torque for those is 40 N*m or 25 ft. lbs. This is a considerable amount of torque. What did you use to tighten the lockring? Chances are you just didn't get it tight enough.
However, I wouldn't expect a Next branded bike to use a cassette. Can you post a picture of the problem area?