Known Issues - Windows 10 Mobile Insider Preview Build 14951
Signing into apps such as Feedback Hub, Groove, MSN News, etc. with your Microsoft Account if you sign out or get signed out of these apps will not work. If this happens to you and you cannot sign in to Feedback Hub, you can send us feedback via the forums.
When rebooting a phone on this build and Build 14946 from last week, the device appears to get "stuck" on the blue Windows logo screen during the boot cycle. We have identified 2 bugs causing this issue and working to check in fixes soon. However, being "stuck" is only temporary. Depending on which device you have, your phone may be in this stuck state for 20-30 minutes, but it will eventually progress to the Lock screen. There is no permanent impact from these 2 bugs, just an unexpectedly long boot time. Please don't reset your device! It'll take longer to reset the device than it will to wait for the boot cycle to complete.
Excel Mobile will freeze after adding sheets and eventually crashes.
Insiders who have configured a data limit on their phone may get into a state where they receive frequent notifications about having exceeded this limit. To resolve the issue, please go into Settings > Device & Network > Data Usage and remove and recreate your data limit settings.