java.lang.IllegalArgumentException: Illegal base64 character 5b

This exception is thrown when attempting to decode a Base64 encoded string and encountering an illegal character. In this case, the illegal character is '5b', which represents the '[' character.

To resolve this issue, you should ensure that the input string is a valid Base64 encoded string. This means that it should only contain characters from the Base64 alphabet, which consists of 64 characters: A-Z, a-z, 0-9, + and /. If the input string contains any other characters, it is not a valid Base64 encoded string and you will need to fix it before attempting to decode it.

If you are unsure where the input string is coming from, you may want to check the source of the string to ensure that it is being generated correctly. It is also possible that the string has been corrupted or modified in some way, so you may want to try generating a new string and decoding that instead.