Problems with Activity data when upgrading German-Version CiviCRM 1.4

In upgrading CiviCRM 1.4.5789M Drupal PHP5 to v1.5 rev7470, I encountered problems with reading the old entries for phone call logs.
When I looked up those, I got the message: "Could not find callback CRM_Activity_Form_Phonecall::showCallDetails". This function name is also found in the data base but not in the source code.
When I created a new phonecall log entry with CiviCRM 1.5, I got not one but two entries, which, however linked to a different, older log, not the one I just created.

To further investigate the error, I installed a blank version of Civicrm without my data, but with the sample data.
I created an phone log entry and tried to read it, leading to an error: "Cannot display Activity History details since activity type for this activity has been deleted."
Once I switched of the German Localisation, it worked fine.

I have no Idea, whether the two problems are related.
Maybe there is a problem with english language description of "phone call" hardcoded?