Connection to SMB server fails with message "Operation not permitted"

Boxee Box is connected via cable to the local network. The fileserver is using FreeNAS (based on FreeBSD) and provied filesharing via Samba. If authentication on the fileserver is completely switched off, Boxee can connect and play shared media files without any problems. However, if I use authentication on the server the connection fails with error message "Operation not permitted". This happens if I provide a valid login, as well as if the particular share for media is public (allows guest access). The samba server was tested with MacOSX and Windows clients and works in all scenarios as expected. There are various open tickets stating similar problems with other NAS systems, therefore I assume there is a bug in Boxee's Samba client implementation.

A debug logfile with different failed login attempts is provided as an attachment.