Unrar shipped with nzbget is compiled by me from official unrar source code. Because this source code doesn't come with good documentation about how to compile it properly for each system, it is well possible that I have used wrong compiler settings affecting unicode compatibility. Moreover it's compiled using a certain cross-compiling toolchain and results may be not 100% compatible with your system.
You can try compiling current version of unrar specifically for your system using compiler provided by system vendor.
Another possibility is to ask the NAS vendor to update their unrar.