The input is not a valid Base-64 string as it contains a non-base 64 character, more than two padding characters, or a non-white space character among the padding characters.
at System.Convert.FromBase64String(String s)
请问这个是什么原因引起的,之前一直是成功的,而且设置的是隔一段时间取一次,期间还有成功下载的,求原因 ?