Microsoft has just released an update to fix a serious app-related bug in the latest Windows 10 Insider Fast Ring build version 14332.
Specifically, the aforementioned bug is in the Mail and Calendar app. Unlucky users who installed Build 14332 might see their mail account disappeared from the app, as well as all of their emails. The bug reportedly affects both PC and Mobile versions of the build, and was common among Insiders.
According to Richard Hay of WinSuperSite, he was able to fix the issue with "a phone restart and poking around in the app"; unfortunately it was not the cure for all users. Fortunately, however, Microsoft responded quickly with a new update for the app pushed to the Windows Store just this morning, whose existence was made known through a tip from Twitter user @MarkXA.
While this sounded like a serious bugs, such occurrence should be expected - and indeed is the whole reason behind - the Windows Insider program, especially the Fast Ring. Microsoft's receptiveness and speed in fixing the problem should give early testers some comfort however, and is a good sign for when more stable builds get released. Besides the bug, build 14332 also brought several new features, including improved battery life for Connected-standby supported devices, Cortana and Office 365 integration, and improvement to the Command Prompt.