IS there any error messages from NI-FBUS server? Could you try to connect NI-FBUS server with other OPC clients like Server Explorer, it would provide more message about the error. If you still have problems, please send e-mail to "fieldbus.support@ni.com" for further support.
On the other hand, I suggest you upgrade your NI-FBUS 2.3.5 to version 2.3.6, which is compliant with OPC 2.0, as well as some bug fixes. You can visit URL below to download it:
http://digital.ni.com/softlib.nsf/websearch/20DF560BEE09622386256BD50054559D?opendocument&node=132060_US
Please make sure uninstall the NI-FBUS 2.3.5 first then install this package.
Hope it helps.