Windows Server 2008 is more or less equivalent to Vista-era Windows, which means it's not supported for Terragen 4. That doesn't mean it won't work, it just means we use some features with an OS older than Windows 7 might not have.
The first thing I'd look at in a server environment is the graphics card and drivers though, since they often have minimal/no GPU. Were you rendering with the commandline version? I guess not because if I remember correctly you use only the free version?
- Oshyan