Password encryption deycrption UTF8 Encoding String conversion doesn't work

I wanna save a password encrypted in a local database (on windows phone). For that I first have to encrypt the password with this.

byte[] passwordInByte = UTF8Encoding.UTF8.GetBytes(clearPW);
byte[] protectedPasswordByte = ProtectedData.Protect(passwordInByte, null);
return UTF8Encoding.UTF8.GetString(protectedPasswordByte, 0, protectedPasswordByte.Length);

When I use this I get the encyrpted password like this:

"\0賐�ᔁᇑ窌쀀쉏\0쐮�毸䉯놜侫䅛\0\0\0\0昐\0Ā\0 \0⎛폳띪ꌾၜჸ㸴뀲േꞪ賂ﯛ䎳웠姒\0\0耎\0Ȁ\0 \0ｔƽ팃ல�쿄갗ꪄ᫓\0꒱௴贒⊡噷�ʥ魚@\0챞Ċ뽨뒸縉菒Æ�≁ᦹ諻⋤떵孵ר䒹춳✴頻ľ�뚿㠔꾹놼ɝ⍔" And the protected passwordPasswordByte is has a length of 230 bytes.

My decryption function looks like this:

byte[] encryptedBytes = UTF8Encoding.UTF8.GetBytes(encrytpedPW);
byte[] passwordByte = ProtectedData.Unprotect(encryptedBytes, null);
return UTF8Encoding.UTF8.GetString(passwordByte, 0, passwordByte.Length);

Now the enycrptedBytes array is 290 bytes length and the password cannot be even decrypted because the Unprotect function throws the error "Invalid data"

But everything works fine in case I use the Convert.BaseString64 functions like this:

encryption:

byte[] passwordInByte = UTF8Encoding.UTF8.GetBytes(clearPW);
byte[] protectedPasswordByte = ProtectedData.Protect(passwordInByte, null);
return Convert.ToBase64String(protectedPasswordByte, 0, protectedPasswordByte.Length);

decryption:

byte[] encryptedBytes = Convert.FromBase64String(encrytpedPW);
byte[] passwordByte = ProtectedData.Unprotect(encryptedBytes, null);
return UTF8Encoding.UTF8.GetString(passwordByte, 0, passwordByte.Length);

Does anybody of you has an idea what is so wrong about the UTF8 encoding? Because the first way is the recommended was by MSDN.