HP Proliant ML110 G6 - standby (sleep) not available

Hello, I work in a small no-profit organization, and we got as a donation an old, but perfectly working ML110 G6 to use in our lab for education activities of unemployed people. Since the orignal owner bankrupt I don't have access to official support or download of stuff like bios update.

I've installed Windows 10 on it and it works flawlessly (with a NVIDIA video card we bought), even if I guess it's supposed to work with Windows Server. Only glitch: sleep is not available neither in start menu nor as an option in power management control panel. Tried as usual under "choose what power button does" and I have only "fast boot", "hybernate" and "lock". It's weird because I can hybernate without problems, but not put the machine into sleep.
I've installed latest video card drivers as suggested elsewhere, but nothing changed.
Is there some driver I can install to fix it, or is it an hardware incompatibility because it's a server?

It's not a major issue, we can live without it especially because hybernation works, but if it possible I'd like to fix it.

Thanks in advance!