The next version of Windows Server is coming this fall, and in the Long-Term Servicing Channel, it will be called Windows Server 2019. It comes from the Redstone 5 development branch, and Microsoft released another Insider Preview today, bringing the build number to 17666.
It replaces build 17650, which was released only a few weeks ago. Interestingly, Microsoft seems to be releasing Windows Server Insider Preview builds almost as frequently as it's releasing consumer builds to the Fast ring these days. They're certainly arriving more often than client builds in the Slow ring, which haven't even arrived for RS5 just yet.
Microsoft also released Windows 10 SDK Insider Preview build 17666 today, bringing the SDK, Server, and client to a parity with build numbers.
Sadly, the company hasn't actually announced either of the new builds. There's still no blog post, so there's no way to find out what's new. This actually isn't very uncommon, and we'll probably learn more over the next day or so.
Update: Dona Sarkar has announced the Windows Server 2019 Insider Preview build 17666 via Twitter with a link to the Windows blog: