If you have upgraded to Handheld Contact version 4.1 there is a bug in the app regarding the Calendar.

The Calendar default view is to show 'All users'.  This is where the bug lies and causes problems.  It appears to be found only within non-English devices.

To correct, go to the Handheld Contact Settings, choose Calendar and click on 'Select User'.  This will be currently on 'All users'.  Change it to any other ACT! user (usually your own name) and press Done.  Go back to the Calendar view and you will be able to see the activities again.

This bug will be fixed as soon as possible.  We have submitted the update to Apple and are awaiting approval.  Once it is live you can upgrade from the Apple App Store.

UPDATE:  This was updated by Apple and released on Friday June 29th.  You can now upgrade your device to the latest version.