This OS is not supported and for supported OS, refer the product guide.
yes you're right. I didn't see you already answered my question, so I did a test install on 64 Bit - and it works. I think it was not clear for me in the product guide as there was mentioned
• Windows 2008 R2 SP1, or
• Windows 2008 SP2 (64-bit), or
(but maybe there ist no 32Bit of "R2"?). However - 64 Bit did it (and that also fits to the "6 GB RAM" requirement...).