Would you have any idea why?
No, I don't, but I do know it shouldn't do that.
First, try re-downloading the TurboActivate.dat from your version page in LimeLM. Then replace the existing TurboActivate.dat file. Does that fix it?
Have you tested our example C# app? Change the GUID, plop the TurboFloat.dll in the Debug folder alongside your TurboActivate.dat file and then run the app. Do you get the same exception there?