No graphs and CSV content for agreements with GOKb titles

Overview:
The eUsage-reports plugin does not work with agreements that contain agreement lines linking GOKb-related titles. While matching in eUsage seems functional, there are no graphs or CSV file contents available in Agreements.

Steps to reproduce:
1. Connect GOKb to the Local KB.
2. Create an Agreement with agreement lines that link to titles or packages imported from the GOKb
3. Create a corresponding UDP in eUsage, harvest or upload Counter reports and link the UDP from the Agreement.
4. Start the title matching from eUsage.
5. Start Agreement analysis from Agreements, view graphs and CSV file.

Expected results:
Titels in COUNTER reports and agreement lines should match.
Graphs and CSV files should have content.

Actual results:
The matching is successful, but neither graphs nor CSV do contain any content.

Further information:
The problem does not occur for titles that have been manually uploaded to the Local KB via a KBART file. 
The problem does occur also for manually uploaded titles once there also is a GOKb version of the title that has been matched within the Local KB.

I suppose that there is some kind of difference in the Local KB between "manually uploaded only" titles vs. GOKb titles. 
The group only tested with KBART-sourced data when you developed the functionalities.
They did this, because the manually uploaded data was much easier to control and thus a seemingly better test set. In retrospective, this unfortunately was a systematical error in the group's testing, as the functionality of course was always designed to support Local KB titles in genreal, so manually uploaded AND harvested titles.
As most institutions do use GOKb in their productive environments, the bug causes almost all agreements to not show any data.