>But it's not clear where and how you are seeing this message? IAC, it would seem that this particular .FromBase64String() is not causing the problem.
>FWIW there are two exception messages for .FromBase64String() - the one you see and "Invalid character in a Base-64 string". The latter takes precedence so you can be confident that there are no invalid characters in the string.
>See help for valid length - basically the number of non-whitespace characters must be zero or a multiple of four.
>If you are creating initVector somewhere how are you doing it?

Here is the Decrypt() method:

        Public Function Decrypt(ByVal data As String, ByVal userKey As String, _
         ByVal alg As EncryptionAlgorithm) As Boolean
            Dim tok As [String]() = data.Split(New Char() {" "c})
            Dim initVector As [String] = tok(0)

            data = tok(1)

            ' Set up the Decryptor object
            Dim dec As New Decryptor(alg)

            ' Set the Initialization Vector
            dec.IV = Convert.FromBase64String(initVector)

            Dim key As Byte() = Encoding.Default.GetBytes(userKey)

            ' Perform the decryption
            If Not dec.Decrypt(Convert.FromBase64String(data), key) Then
               Return False
            End If

            cDecrypt = Encoding.Default.GetString(dec.oByte)
            Return True
        End Function

I adjusted it to remove the Try Catch so it would be easier to follow it.