As I suggested you can try replacing the XStream library included in the SmartFoxServer release with a new one.
I've tested using SFS2X 2.11 and removing the XStream jar, then I downloaded the latest XStream 1.4.9 from their website:http://x-stream.github.io/download.html
along with the XMLPull library found in the "optional download" section. I copied the two jars to SFS2X/lib/ and started the server which didn't complain
I've tested the AdminTool which does heavy use of XStream and everything seems to be working fine.
Hope it helps