If you're on Windows 10 version 1903 and you're planning on upgrading to the latest Fast ring build via the Fast ring of the Windows Insider Program, you're in for some bad news. Microsoft discovered a bug in build 18362.53, the latest 1903 update, which prevents you from installing build 18875, which was offered to the Fast ring last week.
Because of this, Microsoft has stopped offering the build to those on build 18362.53. The issue only affects 19H1 to 20H1 upgrades, so if you're already on 20H1, you can still gget the build. In fact, you should still be able to get it if you're on an earlier, production version of Windows 10. If you really want to get 20H1 right now, an easy enough workaround would be to roll your PC back to a production version of the OS, and switch to the Fast ring.
The blog post also said that the Windows Insider team won't be flighting any new 20H1 builds until this is fixed. The team says that it's hoping that's done by the end of the week, but there are no guarantees.
The fix will likely be for build 18362, rather than for the 20H1 build. Since it's only the one update that's affected, it seems like a cumulative update will do, rather than a full new 19H1 build.