The mismatch error is likely because the task form does not have a control lngEmplID and you're call the function with null, i.e. Call Forms("crm_frm_Tasks_Assigned").SetFormRecordSource(null).
If there's a control on the Main form called lngEmplID you can call this function with:
Call Forms("crm_frm_Tasks_Assigned").SetFormRecordSource(Forms!frm_Main!lngEmpID)