If the 1803 update of Windows 10 fails repeatedly with the error code 0xc1900101 and if you are using full system encryption with Veracrypt 1.22 or older, you should upgrade to the latest beta release or the latest stable release if 1.23 or later is available.  Excerpt from the changelog of 1.23 Beta 2:

Changes between 1.22 and 1.23-BETA2 (5 July 2018) :
  - Windows:
     * Add compatibility of system encryption with Windows 10 upgrade using ReflectDrivers mechanism.

If this error occurs, after the update restart you will end up in some kind of recovery menu. If you select "continue with the current windows version", the computer will reboot and the previous non-updated windows will be started. In  the event log (eventvwr)  under Windows Logs/System there should be at least one error with WindowsUpdateClient as source with the event ID 20 and code 0xc1900101 in the text.

The discussion in this thread (German) was not helpful as no logs in this %windir%\Panther directory existed.