I have also tried the latest version released, but still I'm using version 1.0.20063.301.
Steps to reproduce:
0) Ensure you have .NET 1.1 and .NET 2.0 Framework installed
1) Create standard Windows Forms Application
2) Put C1DropDownControl on a Form and show Modal Button on it
3}Configure app to use .NET Framework 1.1 and run it, everything is OK
4) Configure app to use .NET Framework 2.0 and run it, the mentioned exception occures
To configure .NET 1.1 FW app to run on .NET 2.0 FW include these lines in your .config file
<startup>
<supportedRuntime version="v2.0.50727"/>
</startup>
Sincerely
Miroslav Galajda