Microsoft is nearing the public release of new major updates for Windows 10 as well as Windows Server, and thus the release cadence for Insider builds has been speeding up. Today, those using the consumer-oriented Windows 10 got build 18361 in the Fast ring, and those running Windows Server are getting build 18356.1.
Interestingly, the blog post notes absolutely no changes since the last preview build, which was number 18346. There are no new features or bug fixes, and the list of known issues remained the same, too.
The only thing that seems to be new here is that you can no longer activate this version of Windows Server using the keys Microsoft usually provides, and you'll need your own valid activation key. Presumably, this means Microsoft believes the release is very close to the final version and doesn't want users abusing the ability to activate a copy at no cost. Nonetheless, you can test this new release without activating it.
If you're interested, you can head to the Windows Insider website to download the latest build of Windows Server, as well as the corresponding builds for the Language Packs and the App Compatibility Feature on Demand (FoD). You can also download the recently released Windows Admin Center 1902.