This error is probably not due to yDecode - even though there are limitations in the demo it should not produce "unknown error" - this error is a result of something Outlook Express did on its own. The most common error associated with yDecode is "server cannot be found" usually produced when you don't run yDecode but try to open newsgroups (this is simply how yDecode operates) but obviously you have yDecode up and running correctly and configured well.
As the information on Microsoft site suggests:
http://support.microsoft.com/kb/241256800c013f error could happen due to a limitation of Outlook Express having a limit of not being able to join more than 255 parts of a multipart post. That said, you may also want to know that yDecode can decode multiparts automatically - if you enable multipart decoder it will automatically scan posts you download for multiparts - detect if one of them is a multipart and finally when you click on part #1 it will download entire post as a sequence automatically joining parts into a single post. Of course, yDecode itself doesn't have any limit on how many multiparts a post consists of. It may easily join 10 or 1000 headers into a single post all automatically.
You might want to inform yourself about how multipart decoder works - please read the help file regarding this or just follow up this post if you have any further questions.
