Hi iggyiggy.
I have a Sansa as well and am having the same problem. When I run the Library Server on my network srever, I use the "Search for Library Servers" option under the File-Library menu on each of my networked home computers. Everything works fine when playing music and watching videos.
However, when I try to to Sync music on my Sansa...everything seems to transfer okay. But when I look at the Filename column in MC13, the files were all placed in the DATA folder on my Sansa, the file extensions were removed and somekind of number sequencing is placed on the end of the filename. I also have a iriver Clix 1.0 that does the same thing. And it is not just music files that get transferred to the DATA fiolder, all media files (pictures, video, etc.) are all placed there.
Also, when I Sync music to my Sansa and Clix using my server (where the Library Server) is running, I have no problems what-so-ever. All the files are placed in the correct folders.
There has to be something wrong with the way MC13 syncs to these handhelds when using the Library Server is my guess. I placed my Sansa to both MTP and MSC modes and the same result happens. I even played around with the Handheld settings in MC13 and had no luck with any of them.
I never had this problem in MC12 using the same setup I have currently.
I started a thread here...
http://yabb.jriver.com/interact/index.php?topic=51783.0 ...but noone apparently is replying or having the same issues as us.
Hopefully someone else out there has a solution other than going back MC12.