Critical issue with mass change "change owner" in calender list in 6.4.0

If you go to the calender view and you multi-select multiple calendar entries and you go to action "change owner" The operation will take ages and will not return. Also the logs on the server do not show any issue.

Sadly, and the biggest issue:

• the calendar objects are still there, but all references and all data entered in the calendar entries are set to null or nothing.

How to reproduce?

• open 3-4 calendar entries in new tabs (to have them for later compare)
• go back to the calendar list view
• multiselect again the previously opend 3-4 cal entries
• do a change owner operation and change to any owner
• open any of the changed calendar entries and take a look into the entries. All references to Leads or any other object and all text fields are lost.

Here maybe a helpful discussion: https://discussions.vtiger.com/index.php?p=/discussion/181636/mass-update-with-change-owner-on-calendar-list-deletes-every-details-of-the-event#Item_3