The .NET framework is a software platform from Microsoft.

Citavi 5 and 4 require a working installation of .NET framework 4.5.
Citavi 3 requires a working installation of .NET Framework 3.5.

Repairing the .NET framework using Windows

On the Start menu, type cmd.exe. Right-click the only search result (cmd.exe) and then click Run as administrator.
Type sfc.exe /scannow and press Enter. The System File Checker will run, which may take a while.

Repairing the .NET framework using the Microsoft Fixit-Tool

In rare cases, the System File Checker doesn't help, you cannot reinstall .NET, and/or the error recurs. If this happens, please use the Microsoft .NET Framework Repair Tool which can be downloaded from the Microsoft Knowledge Base.