Did you buy your XL brand new or pre-owned.. if its Pre-owned, someone may have made a mess of updating it...
You can download the latest "WindowsDeviceRecoveryTool" and try to Roll-Back to the last OS.. if that fails.. Im afraid youll have to flash the rom for your regon again.. go here:Tutorial | Microsoft Lumia devices and Nokiaâ€¦ | Windows 10 Development and Hacking
If your at all Unsure about flashing.. DONT do it, its not for the faint of heart..
My XL was BNIB, but still Pre-owned, first thing I did was used the REcovery Tool, then updated to 420. I know Windows is a locked platform and not as "hackable" as Android, but I still Erase and Re-Flash any Pre-owned phone I buy