Failed to compare two elements in the array

When opening a project you receive the following error message: 

An unknown error occurred while opening the project. The internal error message is: 
Failed to compare two elements in the array.

Cause

Unknown

Solution

1. Close Citavi.

2. Locate the "ProjectCache" folder by copying the following folder path to the Windows Explorer address bar (screenshot):

%localappdata%\Swiss Academic Software\Citavi 6\

3. Rename the "ProjectCache" folder to "ProjectCache-BAK".

4. Start Citavi.

5. Open your cloud project.

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