Hi,
Usually light optimization fail if time limit exceeds. Execution time will depend on number records in the cube. Try to analyse the record growth and set the time limit.
You can set time limit from RZ11 for rdisp/max_wprun_time to avoid short dumps. When you start light optimization system will change the load behaviour of infocube from Data loading not allowed to planning not allowed . This will be changed back only up on the success of optimization. Due to this changes system does not allow users to do normal operations if optimization process fails.
My advise is do not change load behaviour manually if optimization fails. Try to finish the job by changing time limit.
Thanks,
Raju