Yeah, that is a framework issue. I also hate to give you bad news, but I don't believe that version of Windows Server supports .net
I am looking into other options though, in order to support ServUO on older OS' that don't support .net 4.5. I'd really hate to drop support for 4.5, as we do use at least one feature that is specific to 4.5. I haven't had time to test, but I'm hoping that the newest Mono will run on these OS' with no .net 4.5 support (no promises.)
The major issue is because 4.5 wasn't a new framework, but an in-place upgrade. Meaning it replaces existing binaries from the previous version. The only reason Microsoft won't support other OS' is because the kernel for these old operating systems don't support some of the new stuff in .net 4.5.