What is the error? It should not fail to convert either character, they are both in teh default base64 character set:
http://en.wikipedia.org/wiki/Base64[
^]
When I try, it works fine:
string unicode = "ABCDEFG+/abcedfg";
byte[] ascii = System.Text.Encoding.ASCII.GetBytes(unicode);
string base64 = Convert.ToBase64String(ascii);
byte[] convertBack = Convert.FromBase64String(base64);
string regenerated = System.Text.Encoding.ASCII.GetString(convertBack);
Console.WriteLine(unicode);
Console.WriteLine(regenerated);
How is your code different?