It's been over two months since Microsoft released the last build of Windows Server to Insiders, in the form of build 19551. Now, the hiatus has ended with the release of build 19603, which aligns the build number with that of last week's Windows 10 build for Insiders in the Fast Ring. It's also the second Windows Server build that's part of the next development cycle following the 20H1 update, which is not generally available yet.
While the new build is available for download, Microsoft hasn't published a blog post for it, so your guess is as good as ours as to what's new in this release. Windows 10 development has been relatively slow in the Fast ring, so it's likely that there aren't a lot of major changes for Windows Server either. We'll update this article when a blog post is live.
Many times, Windows 10 SDK builds also get released on Tuesdays, but that doesn't seem to be the case today. Only the Assessment and Deployment Kit (ADK) has been updated to build 19603, while both the SDK and WDK are still on build 19592. The HLK is even further behind, on build 19041, the same major build number as the Windows 10 20H1 update.