The error message is accurate.  That string has empty label segments in it, which RFC 5890 defines as an error on encoding.  There is no such error defined for decoding, so that doesn't raise an error.

I don't see anything wrong with the error message, it includes the same one as raised in python2.  Perhaps you are confused by the error chaining introduced in Python3?  The second part of the traceback is coming from the encoding machinery, while the first part lets you know where in the encoder the error was raised.  In this case having both doesn't provide much additional information, but if one was debugging a codec or the error were coming from inside an application, it would.