Mass Update Task Status not working

Mass Update Task Status not working

Dear all,
I need to mass update the task status of a few thousand activities to "Completed". Two problems occur that I hope you can help me with.

1. Only 100 Records a time can be selected for mass update
2. Even after mass update the activity status to "Completed" the task seems completed but still shows no "Closed Time". Now the tasks are still visible as Open, even though the status is completed....

Please help me on what I need to change to make sure that a Closed Time is set.

Best,
Joachim