Base64: java.lang.IllegalArgumentException: Illegal character

Your encoded text is [B@6499375d. That is not Base64, something went wrong while encoding. That decoding code looks good.

Use this code to convert the byte[] to a String before adding it to the URL:

String encodedEmailString = new String(encodedEmail, "UTF-8");
// ...
String confirmLink = "Complete your registration by clicking on following"
    + "\n<a href="" + confirmationURL + encodedEmailString + "">link</a>";