I could not identify any problems with the code you are using. On my end both methods are called properly. The problem might be in the encryption/decryption methods. Have you tried setting breakpoints and debugging the Convert methods? You can check the input parameters' value as well as the values returned by the Encrypt/Decrypt methods. This might help you identify the problem.
If you think the problem is on our side, then I suggest you to open a new support ticket and send us a sample project which reproduces the issue. This will let us investigate this case in detail and provide you with an appropriate answer.
Hope this helps. Feel free to ask if you have any further questions.
the Telerik team
Q1’11 SP1 of RadControls for WinForms is available for download
; also available is the Q2'11 Roadmap
for Telerik Windows Forms controls.